Find the flaw in the proof

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Find the error in the proof.

This is the question:

Theorem: Every positive integer is equal to the next largest positive integer.

Proof: Let $P(n)$ be the proposition that $n=n+1$.To show that $P(k)$ implies $P(k+1)$, assume $P(k)$ is true such that $k=k+1$. Adding 1 on both sides we get $k+1=k+2$. Which is nothing but $P(k+1)$. Thus, $P(k)-P(k+1)$ is true. Hence $P(n)$ is true for all positive integers $n$.

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Is it $P(1)$ true? Is it $1=2?$

When you apply induction you need two things:

  • $P(1)$ holds, and
  • If $P(n)$ is true then $P(n+1)$ holds.

In your case the first condition fails.

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The how of a Proof by Induction works is often explained by analogy to "showing that a chain of dominos will fall."

You have successfully shown that if any tile is tipped, then the subsequent tile in the chain will also tip. $$\forall n\geq 1: p(n)\to p(n+1)$$

However, you have neglected to show that the first tile in the chain can be tipped, rather than being set up firmly against a wall; as in fact is the case. $$\require{cancel} \xcancel{p(1)}$$