[thwd]

> We will show, by induction, that [for all sets] of n horses, every horse in [each] set has the same color.

> Now [assume that] for all sets of n horses, every horse in [each] set has the same color.

QED.

This is just tautology. No further analysis needed, really.

(If your proof includes your hypothesis as an assumption, then it must be a proof by contradiction.)

EDIT: Before you refute, read again carefully. The assumption _is_ the hypothesis. It is not existentially quantified or set in the base case. It is the entire, universally quantified hypothesis.

[tomtomlapomme]

This is how induction proof work. You prove that something is true for a base (n = 1) case then you prove that if it is true for n it has to be true for n + 1. Therefore it is true for any n.

The flaw is that the second part of the proof requires n >= 2

[thwd]

I understand and agree but read carefully. The assumption _is_ the (entire) hypothesis. It is not existentially quantified or set in the base-case. It is the entire hypothesis.

[ashtonbaker]

You should read “n” as “some arbitrary n” in the quote you posted. It’s not an assumption for all n.

edit: sorry, you’re right that the wording is a bit sloppy. The “n” in the first quote is “for all n” and the n in the second quote is “some specific n” or “some arbitrary n”. They’re not meant to be the same statement. I don’t think it’s a very carefully written article.

[shkkmo]

The assumption that the case holds for n while the goal is to show the case holds for n+1. It is not well worded to make clear the intent of making an assumption about modal possibility. However, to see it as begging the question requires imposing a statement of modal necessity that simply isn't there.

[jstanley]

I thought the same thing as you at first, but you need to read more carefully.

The proof is showing that it is true for n=1, and then showing that if it is true for n (the part where we "suppose it is") then it is true for n+1, proving by induction that it is true for all n >= 1.

[clktmr]

Yet, that assumption is not needed for the proof and can simply be removed. I think OP just wanted to say "what follows is the proof for the induction step".

[jm_l]

Proof by inductions often involve showing that Pn implies Pn+1. That is, that a statement's truth for n implies it's truth for n+1. That's what's being done here, and it's a perfectly valid part of this type of proof.