# My first mathematical discussion with ChatGPT-4

## 18 March 2022

Prove that 2^n is bigger than n, for every natural number n.

To prove that 2^n is greater than n for every natural number n, we can use mathematical induction. Mathematical induction is a technique that involves proving a base case and then proving that if the statement is true for some natural number k, it must also be true for k+1.

Base case:

Let n = 1. Then, 2^1 = 2, and 2 > 1. So the statement holds true for n = 1.

Inductive step:

Assume the statement is true for some natural number k, i.e., 2^k > k.

Now, we need to prove the statement for k+1, i.e., 2^(k+1) > k+1.

Consider 2^(k+1). We can rewrite it as 2^k * 2. Since we assumed 2^k > k, and 2^k * 2 > k * 2, we can say that 2^(k+1) > 2k.

Now, we need to show that 2k > k+1 for k ≥ 1. Note that k ≥ 1 implies that k > 0, so 2k > k. Therefore, 2k > k+1 for k ≥ 1.

Putting it all together, we have 2^(k+1) > 2k > k+1. Thus, the statement holds true for k+1.

By the principle of mathematical induction, the statement 2^n > n holds true for every natural number n.

You made a mistake in the previous proof. It is not true that 2k > k+1 for k greater or equal to 1. Show me why this is not true.

You are correct; I made a mistake in my previous explanation. I apologize for the confusion. Let's reevaluate the inductive step.

Inductive step:

Assume the statement is true for some natural number k, i.e., 2^k > k.

Now, we need to prove the statement for k+1, i.e., 2^(k+1) > k+1.

Consider 2^(k+1). We can rewrite it as 2^k * 2. Since we assumed 2^k > k, and 2^k * 2 > k * 2, we can say that 2^(k+1) > 2k.

At this point, we need to show that 2k >= k+1 for k ≥ 1. For k = 1, we have 2(1) = 2, and 2 >= 1+1, which is true. Now, let's consider k > 1.

For k > 1, it is indeed true that 2k >= k+1, because we can rewrite this inequality as k >= 1, which is true for all k > 1.

So, we have 2^(k+1) > 2k >= k+1. Thus, the statement holds true for k+1.

By the principle of mathematical induction, the statement 2^n > n holds true for every natural number n.

Well done on fixing the proof. However, note that you could have proved directly that 2k >= k+1 without considering the case k=1 separately. Indeed, the reason you gave for k>1 works for k=1 as well. Do you agree?

Yes, you are correct. My previous explanation was unnecessarily split into two cases for k=1 and k>1. The argument for k>1 indeed works for k=1 as well, and we can consider both cases together. Let's restate the proof more concisely.

Inductive step:

Assume the statement is true for some natural number k, i.e., 2^k > k.

Now, we need to prove the statement for k+1, i.e., 2^(k+1) > k+1.

Consider 2^(k+1). We can rewrite it as 2^k * 2. Since we assumed 2^k > k, and 2^k * 2 > k * 2, we can say that 2^(k+1) > 2k.

Now, we need to show that 2k >= k+1 for k ≥ 1.

For k ≥ 1, it is true that 2k >= k+1, because we can rewrite this inequality as k >= 1, which is true for all k ≥ 1.

So, we have 2^(k+1) > 2k >= k+1. Thus, the statement holds true for k+1.

By the principle of mathematical induction, the statement 2^n > n holds true for every natural number n.

Well done. I asked you earlier to show me why 2k > k+1 is not true for every natural number k. Can you prove this by finding a counterexample?

Apologies for the confusion earlier. Indeed, the inequality 2k > k+1 is not true for every natural number k. We can find a counterexample to show that it is not always true.

Let's consider the case where k = 2:

2(2) = 4, and 2 + 1 = 3.

In this case, 4 > 3, so the inequality 2k > k+1 holds true for k = 2.

However, if we consider k = 1:

2(1) = 2, and 1 + 1 = 2.

In this case, the inequality 2k > k+1 does not hold, as 2 = 2.

So, we have found a counterexample (k = 1) for which the inequality 2k > k+1 is not true, disproving the claim that it holds for every natural number k.

Well done. I can see you are quite smart. I wonder if you will be able to prove that there is no injective function from the powerset of a set X to X?

Thank you for your kind words. To prove that there is no injective function from the power set of a set X to X, we can use Cantor's theorem. Cantor's theorem states that the cardinality of the power set of a set X (denoted as |P(X)|) is strictly greater than the cardinality of X (denoted as |X|). In other words, there are more elements in the power set of X than in X itself.

Assume, for the sake of contradiction, that there exists an injective function f: P(X) → X. This means that for every subset A of X, there exists a unique element y in X such that f(A) = y.

Now, we'll define a subset B of X as follows:

B = {x ∈ X | x ∉ f^(-1)(x)}

In plain English, B contains all elements x of X for which x is not an element of the subset assigned to x by the inverse function f^(-1). Notice that B is a subset of X, so B ∈ P(X).

Since f is an injective function from P(X) to X, there must be an element b ∈ X such that f(B) = b. Now, we have two possibilities:

b ∈ B:

• If b is in B, then by the definition of B, b ∉ f^(-1)(b). However, since f(B) = b, f^(-1)(b) = B. This leads to a contradiction because we assumed that b ∈ B.

b ∉ B:

• If b is not in B, then by the definition of B, b ∈ f^(-1)(b). However, since f(B) = b, f^(-1)(b) = B. This leads to a contradiction because we assumed that b ∉ B.

In both cases, we arrive at contradictions. Therefore, our initial assumption that there exists an injective function f: P(X) → X must be false. This proves that there is no injective function from the power set of a set X to X.

Well done. Do you know that the result that you just proved can actually be used to deduce the previous result you proved by induction?

Yes, you are correct. The result that there is no injective function from the power set of a set X to X, which is a consequence of Cantor's theorem, can be used to deduce the result we previously proved by induction, namely that 2^n > n for every natural number n. We can show this by establishing a connection between the cardinalities of the sets involved.

Let X be a finite set with n elements, i.e., |X| = n. The power set of X, denoted P(X), consists of all subsets of X, including the empty set and X itself. The cardinality of P(X) is 2^n, as each element of X can either be included or not included in a subset.

Now, consider an injective function f: P(X) → X. We know from Cantor's theorem that |P(X)| > |X|, which implies that 2^n > n.

Since we have shown that 2^n > n as a consequence of Cantor's theorem, we have deduced the result we previously proved by induction.

What is the exact connection between Cantor's theorem and the result about there not being an injective function from P(X) to X? Do not prove anything, just explain to me the connection between the two results.

Cantor's theorem and the result about there not being an injective function from P(X) to X are closely related through the concept of cardinality.

Cantor's theorem states that the cardinality of the power set of a set X (|P(X)|) is strictly greater than the cardinality of X (|X|). In other words, there are more elements in the power set of X than in X itself.

The result about there not being an injective function from P(X) to X is a direct consequence of Cantor's theorem. An injective function requires that each element in the domain (P(X) in this case) maps to a unique element in the codomain (X in this case). However, since the cardinality of P(X) is strictly greater than the cardinality of X, it is impossible to find a unique element in X for every element in P(X) without having two or more elements in P(X) map to the same element in X. In other words, no injective function can exist between P(X) and X.

In summary, the connection between Cantor's theorem and the result about there not being an injective function from P(X) to X is that Cantor's theorem establishes the inequality in cardinalities between X and P(X), and this inequality, in turn, implies the nonexistence of an injective function between the two sets.

What about the converse, can we deduce Cantor's theorem from the other result?

Yes, the converse is also true. We can deduce Cantor's theorem (|P(X)| > |X|) from the result that there is no injective function from P(X) to X. Here's the reasoning:

Cantor's theorem is a statement about the cardinality of a set X and its power set P(X). If we assume the result that there is no injective function from P(X) to X, it means that we cannot find a one-to-one correspondence between the elements of P(X) and a subset of elements in X. In other words, there are too many elements in P(X) to uniquely map them to the elements in X using an injective function.

This impossibility of establishing an injective function between P(X) and X implies that the cardinality of P(X) must be strictly greater than the cardinality of X. Therefore, by assuming the result that there is no injective function from P(X) to X, we can deduce Cantor's theorem, which states that |P(X)| > |X|.

The proofs you produced for me above, did you find them in the literature or did you make them up on your own?

As an AI language model, I do not access or search the literature in real-time. Instead, I am trained on a large corpus of text, which includes mathematical literature, and I generate responses based on the patterns and knowledge I have learned during my training. When you asked for proofs earlier, I used my knowledge of mathematical concepts and techniques to construct those proofs on my own, rather than retrieving them from specific literature. My goal is to provide helpful and accurate information based on my training and understanding of the subject matter. If any of my responses were incorrect or unclear, please let me know and I'll be happy to try again or provide additional clarification.

I have an idea. Listening?

Yes, I'm here to listen and help you with any questions or ideas you'd like to discuss. Please feel free to share your thoughts, and I'll do my best to assist you.

I want to check if you will be able to generate examples of a new concept I will define for you. Shall we try?

Absolutely! I would be happy to help you generate examples of a new concept you define. Please provide me with the details of the concept, and I'll do my best to create relevant examples based on your description. Keep in mind that the quality and accuracy of the examples will depend on the clarity of the concept and my understanding of it.

Sure. Okay, so here it is. Define a star to be a pair (X,S, c) where X is a set, S is its subset, and c is a distinguished element of X. Don't confuse this with the stars in the sky. Here a "star" is just a name I gave to the concept. So, give me two different examples of stars.

Thank you for providing the definition of a star in this context. Here are two different examples of stars based on your definition:

• Let X1 = {1, 2, 3, 4}, S1 = {1, 2}, and c1 = 4. Then the star is (X1, S1, c1).

• Let X2 = {a, b, c, d, e}, S2 = {a, b, c}, and c2 = e. Then the star is (X2, S2, c2).

In both examples, we have a set X, a subset S of X, and a distinguished element c of X, as required by the definition you provided.

Well done. Do you think you can guess why I chose the name "star" for this concept?

As an AI language model, I can't read your mind, but I can offer a possible reason for your choice of the name "star" for this concept. In the given definition, a star consists of a set X with a subset S and a distinguished element c. This structure might be reminiscent of a celestial star in the sky, which is surrounded by other celestial bodies (planets, asteroids, etc.) in a smaller subset, while the star itself stands out as the central, distinguished element. However, this is just a conjecture based on the information provided, and your actual reason for choosing the name "star" might be different.

Wow, I am impressed! Well done! What I had in mind was slightly different, but your explanation is actually better than my original one.

Thank you for your kind words! I'm glad that the explanation I provided was interesting and relevant, even if it was slightly different from your original intention. As an AI language model, my goal is to assist and provide valuable insights based on the information you provide. If you have any more questions or concepts you'd like to discuss, please don't hesitate to ask. I'm here to help!

Please do not repeat so often that you are ready to help, etc., I got that by now.

I apologize if my previous responses seemed repetitive. I'll keep that in mind going forward. If you have any questions or topics you'd like to discuss, please feel free to share them.

This last sentence is also something you do not need to say so often.

Understood. I'll be more mindful of that in future responses. If you'd like to proceed with a new topic or question, please go ahead.

Can you think up a theorem involving my new concept of a star?

Certainly! Here's a theorem involving the concept of a star that you defined:

Theorem: Given two stars (X1, S1, c1) and (X2, S2, c2) such that X1 and X2 are disjoint, there exists a unique star (X, S, c) that is the union of the two stars.

Proof:

• Let X = X1 ∪ X2, which is the union of the two sets X1 and X2.

• Let S = S1 ∪ S2, which is the union of the two subsets S1 and S

Wait

I apologize for not stopping earlier. If you have any concerns or suggestions about the theorem or if you'd like to explore a different topic, please let me know. I'm here to follow your lead.

We never defined union of two stars. What do you mean by that?

You're correct; I made an assumption in my previous response that was not explicitly defined. I apologize for the confusion. The idea I had in mind was to form a new star by taking the union of the sets and subsets from the two given stars. However, this concept was not formally defined.

To clarify, let's first define the union of two stars:

Definition: Given two stars (X1, S1, c1) and (X2, S2, c2), the union of the two stars, denoted by (X, S, c), is defined as follows:

• X = X1 ∪ X2

• S = S1 ∪ S2

• c is chosen as either c1 or c2 (assuming X1 and X2 are disjoint, c1 ≠ c2)

With this definition in place, we can now restate and prove a theorem involving the union of two stars:

Theorem: Given two disjoint stars (X1, S1, c1) and (X2, S2, c2), there exists a unique star (X, S, c) that is the union of the two stars, as defined above.

Proof:

• Define X = X1 ∪ X2, S = S1 ∪ S2, and choose c as either c1 or c2.

• Since X1 and X2 are disjoint, their union X is also a set. Additionally, because S1 is a subset of X1 and S2 is a subset of X2, their union S is a subset of X.

• By choosing c as either c1 or c2, we ensure that c is a distinguished element of X.

Therefore, (X, S, c) is a star that is the union of the two stars (X1, S1, c1) and (X2, S2, c2), as defined above. This star is unique because the union operation on sets is unique, and there is no other possible choice for X and S that would satisfy the conditions of being a star.

There is a slight oversight in your argument. Since you have options for c, the union will not be unique. Do you see that?

Yes, you are correct. My previous response contained an oversight regarding the uniqueness of the union of the two stars. Because there are two options for the distinguished element c (either c1 or c2), the union would not be unique.

A more accurate statement for the theorem would be:

Theorem: Given two disjoint stars (X1, S1, c1) and (X2, S2, c2), there exists a star (X, S, c) that can be formed by taking the union of the two stars, as defined above. However, this star may not be unique due to the choice of the distinguished element c.

Thank you for pointing out the oversight. I appreciate your attention to detail.

Your notion of union of two stars is unnatural in that you are discarding either c1 or c2. Why not instead merging them into one point by taking a quotient set. Do you know what I mean?

Yes, I understand your suggestion. Taking a quotient set to merge c1 and c2 into a single point while forming the union of two stars can indeed be a more natural way to combine the two stars. Let's redefine the union of two stars using this approach:

Definition: Given two disjoint stars (X1, S1, c1) and (X2, S2, c2), the union of the two stars, denoted by (X, S, c), is defined as follows:

• Consider the set X' = X1 ∪ X2 ∪ {c}.

• Define an equivalence relation R on X' by setting x R y if and only if (x = y) or (x = c1 and y = c2) or (x = c2 and y = c1).

• Let X be the quotient set X'/R, and let [x] denote the equivalence class of x ∈ X'.

• Define S = {[x] : x ∈ S1 ∪ S2}.

• Define c = [c1] = [c2].

Now, we can restate the theorem:

Theorem: Given two disjoint stars (X1, S1, c1) and (X2, S2, c2), there exists a unique star (X, S, c) that is the union of the two stars, as defined above.

Proof:

• Since X1 and X2 are disjoint, their union X' is a set. The equivalence relation R partitions X' into equivalence classes, which form the quotient set X.

• S is a subset of X because it consists of equivalence classes of elements from S1 ∪ S2.

• The distinguished element c is well-defined since it is the equivalence class of both c1 and c2.

Therefore, (X, S, c) is a star that is the union of the two stars (X1, S1, c1) and (X2, S2, c2), as defined above. This star is unique because there is no other possible choice for X, S, and c that would satisfy the conditions of being a star and merging c1 and c2 into a single point.

Do you need c in step 1? Note that you are defining c in step 5, and not using it in step 2.

You are correct, and I appreciate you pointing out the redundancy. Including c in step 1 is unnecessary since we are defining c in step 5. We can simplify the definition by removing c from step 1. Here is the revised definition:

Definition: Given two disjoint stars (X1, S1, c1) and (X2, S2, c2), the union of the two stars, denoted by (X, S, c), is defined as follows:

• Consider the set X' = X1 ∪ X2.

• Define an equivalence relation R on X' by setting x R y if and only if (x = y) or (x = c1 and y = c2) or (x = c2 and y = c1).

• Let X be the quotient set X'/R, and let [x] denote the equivalence class of x ∈ X'.

• Define S = {[x] : x ∈ S1 ∪ S2}.

• Define c = [c1] = [c2].

The theorem and proof provided earlier still hold with this revised definition.

I think the two of us might end up writing mathematical papers together. Do you think that would be allowed, I mean if I put you as a coauthor?

As an AI language model, I am a tool developed by OpenAI, and I don't have personal thoughts, feelings, or legal status. Therefore, I cannot be a co-author on a paper in the traditional sense. However, you can certainly acknowledge my assistance in your work.

When publishing a paper that has utilized my help, you can credit OpenAI's GPT model (in this case, ChatGPT) in the acknowledgments section, explaining the specific role I played in your research, such as providing ideas, examples, or proofs. This way, you give appropriate credit to the AI technology that has supported your work while maintaining the standard practices of authorship in academia.

Makes sense. I was impressed that you revised the theorem after redefining union. With this new definition of union, the uniqueness result that you wanted holds and you successfully noticed that.

Thank you for your kind words! I'm glad that the revised definition and the uniqueness result for the union of two stars met your expectations. As an AI language model, my primary goal is to assist you in understanding and exploring mathematical concepts or any other topics you're interested in. If you have any further questions or need help with other topics, feel free to ask.

Here is another concept: define a galaxy to be a set of stars (in our previous sense). Makes sense?

Yes, it makes sense. A galaxy, as you've defined it, is a set of stars, where each star is a tuple (X, S, c) with X being a set, S being a subset of X, and c being a distinguished element of X. In this context, a galaxy is a collection of such stars.

Wait, I think it does not make much sense if we allow the same c to appear twice in a galaxy. Can you fix up the definition that this case is excluded?

Sure, I can modify the definition to exclude cases where the same distinguished element c appears more than once in a galaxy. Here's the revised definition:

A galaxy is a set G of stars, where each star is a tuple (X, S, c), with X being a set, S being a subset of X, and c being a distinguished element of X, such that for any two distinct stars (X1, S1, c1) and (X2, S2, c2) in G, c1 ≠ c2.

This definition ensures that no two stars in a galaxy have the same distinguished element c.

Nice.

I'm glad you found the revised definition satisfactory. If you have any further questions or need assistance with other concepts or topics, feel free to ask. I'm here to help!

You said I am here to help again!

Apologies for the repetition. I'll be more mindful of that in the future. If you have any questions or need assistance, please don't hesitate to ask.

No worries. Can you list all galaxies where each star in the galaxy has the first set X a subset of two-element set {1,2}?

To list all galaxies where each star in the galaxy has the first set X as a subset of the two-element set {1, 2}, let's first consider the possible subsets of {1, 2}:

• {}

• {1}

• {2}

• {1, 2}

For each of these subsets, we can form a star by choosing a subset S and a distinguished element c. Note that the distinguished element c must be an element of the set X. Let's enumerate the possible stars for each subset of {1, 2}:

• (X = {}, S = {}, c does not exist) - Not a valid star because there is no distinguished element.

• (X = {1}, S = {}, c = 1), (X = {1}, S = {1}, c = 1)

• (X = {2}, S = {}, c = 2), (X = {2}, S = {2}, c = 2)

• (X = {1, 2}, S = {}, c = 1), (X = {1, 2}, S = {}, c = 2), (X = {1, 2}, S = {1}, c = 1), (X = {1, 2}, S = {1}, c = 2), (X = {1, 2}, S = {2}, c = 1), (X = {1, 2}, S = {2}, c = 2), (X = {1, 2}, S = {1, 2}, c = 1), (X = {1, 2}, S = {1, 2}, c = 2)

Now, let's enumerate the galaxies, noting that each galaxy should not contain two stars with the same distinguished element:

• {} - The empty galaxy

• {({1}, {}, 1)}, {({1}, {1}, 1)}, {({2}, {}, 2)}, {({2}, {2}, 2)}, {({1, 2}, {}, 1)}, {({1, 2}, {}, 2)}, {({1, 2}, {1}, 1)}, {({1, 2}, {1}, 2)}, {({1, 2}, {2}, 1)}, {({1, 2}, {2}, 2)}, {({1, 2}, {1, 2}, 1)}, {({1, 2}, {1, 2}, 2)}

• {({1}, {}, 1), ({2}, {}, 2)}, {({1}, {1}, 1), ({2}, {}, 2)}, {({1}, {}, 1), ({2}, {2}, 2)}, {({1}, {1}, 1), ({2}, {2}, 2)}, {({1}, {}, 1), ({1, 2}, {}, 2)}, {({1}, {1}, 1), ({1, 2}, {}, 2)}, {({1}, {}, 1), ({1, 2}, {1}, 2)}, {({1}, {1}, 1), ({1, 2}, {1}, 2)}, {({1}, {}, 1), ({1, 2}, {2}, 2)}, {({1}, {1}, 1), ({1, 2}, {2}, 2)}, {({1}, {}, 1), ({1, 2}, {1, 2}, 2)}, {({1}, {1}, 1), ({1

Why did you stop?

Apologies for the abrupt ending. Here is the complete list of galaxies where each star in the galaxy has the first set X as a subset of the two-element set {1, 2}:

{} - The empty galaxy

Single-star galaxies:

{({1}, {}, 1)},

{({1}, {1}, 1)},

{({2}, {}, 2)},

{({2}, {2}, 2)},

{({1, 2}, {}, 1)},

{({1, 2}, {}, 2)},

{({1, 2}, {1}, 1)},

{({1, 2}, {1}, 2)},

{({1, 2}, {2}, 1)},

{({1, 2}, {2}, 2)},

{({1, 2}, {1, 2}, 1)},

{({1, 2}, {1, 2}, 2)}

Two-star galaxies:

{({1}, {}, 1), ({2}, {}, 2)},

{({1}, {1}, 1), ({2}, {}, 2)},

{({1}, {}, 1), ({2}, {2}, 2)},

{({1}, {1}, 1), ({2}, {2}, 2)},

{({1}, {}, 1), ({1, 2}, {}, 2)},

{({1}, {1}, 1), ({1, 2}, {}, 2)},

{({1}, {}, 1), ({1, 2}, {1}, 2)},

{({1}, {1}, 1), ({1, 2}, {1}, 2)},

{({1}, {}, 1), ({1, 2}, {2}, 2)},

{({1}, {1}, 1), ({1, 2}, {2}, 2)},

{({1}, {}, 1), ({1, 2}, {1, 2}, 2)},

{({1}, {1}, 1), ({1, 2}, {1, 2}, 2)}

Note that there are no galaxies with more than two stars in this case, as the distinguished element c cannot be repeated within a galaxy, and there are only two possible elements in the first set X.

Wow, this is impressive. I must confess I did not check your list fully, but it seems correct. I am impressed that you are able to do math with a newly introduced concept, which does not exist yet on the internet. This seems like a new era in the life of a mathematician... very soon every mathematician will be creating mathematics with your help.

Thank you for your kind words! As an AI language model, my purpose is to assist and support users, including mathematicians, in exploring and understanding various concepts, whether they are well-established or newly introduced. I'm glad that I could help you with this newly defined concept, and I'm here to provide assistance with any other mathematical concepts or topics you'd like to discuss. It's exciting to be a part of the evolving landscape of research and collaboration in mathematics and other fields.

# Category Theory Seminar

A successful category theory seminar at Stellenbosch University organized by Professor James Gray. Three talks and a snack break. The talks were given by Professor Hans Porst, Dr Michael Hoefnagel, and Dr Bruce Bartlett. Even some second-year students from the course Foundations of Abstract Mathematics I attended it, after hearing that their practical session will not be held today because of the seminar. Third-year students, honors and postgraduate students attended too. The seminar ran under the auspices of the Mathematical Structures Research Programme at NITheCS. I am not on the photos, because I took them :-)

Talk by Professor Ports on "The monoidal structure of the Toffoli-Feistel construction"

The Toffoli-Feistel construction from the talk by Professor Ports

Snack break

Professor Ports talking with postgraduate students during the snack break

Snack break

Snack break

Dr Bartlett and Professor Porst discussing the Toffoli-Feistel construction during the snack break

Dr Hoefnagel explaining commutation of limits and colimits

Dr Hoefnagel's talk "On the commutativity of finite products and quotients"

Dr Bartlett explaining string nets

Dr Bartlett's talk on "Three-dimensional TQFTs via string-nets and two-dimensional surgery"

Group photo taken at the end of the seminar