# Is this a correct way to prove by induction?

I understand that most people use the inductive hypothesis, but I find that counterintuitive. Is the below proof correct? In particular, I am concerned with my use of $n$ in (I); is the reason people use another variable, e.g. $k$, for conceptual reasons, or does the use of $n$ create an error in my proof?

\begin{align*} P(0) \land [P(k) \Rightarrow P(k+1)] \implies P(n) \tag{AI} \\ \end{align*}

\begin{align*} \text{When }n = 2, \ \ 2 + 6 + 10 + . . . + (4n - 2) &= 2n^2 \tag{B} \\ 2 + 6 &= 2 \cdot 2^2 \\ 8 &= 8 \\ \end{align*}

$$\begin{pmatrix} 2 + 6 + 10 + ... + (4n-2) = 2n^2 \\ \Big\Downarrow \\ 2 + 6 + 10 + ... + (4n-2) + (4(n+1)-2) = 2(n+1)^2 \\ \end{pmatrix} \tag{I}$$ \begin{align*} \Big\Updownarrow \end{align*} \begin{align*} 4(n+1)-2 &= 2(n+1)^2 -2n^2 \\ 4n+4-2 &= 2n^2+4n+2 - 2n^2 \\ 4n+2 &= 4n+2 \\ \end{align*}

$$\text{B} \land \text{I} \land \text{AI} \implies 2 + 6 + 10 + . . . + (4n - 2) = 2n^2 \text{ for } n > 1 \ \ \square$$

Is this alternate solution correct? I am confident in my reasoning, but am unsure if it is a valid mathematical argument.

Pairing $1$st with $n$th term, $2$nd with $(n-1)$th term, etc., yields $\mathbf{\frac{n}{2}}$ pairs: \begin{align*} 2 + (4n-2) \ \ + \ \ 6 + (4(n-1)-2) \ \ + \ \ 10 + (4(n-2)-2) \ \ + \ \ ... &= 2n^2 \\ 2 + (4n-2) \ \ + \ \ 6 + (4n-6) \ \ + \ \ 10 + (4n-10) \ \ + \ \ ... &= 2n^2 \\ 4n \ \ + \ \ 4n \ \ + \ \ 4n \ \ + \ \ ... &= 2n^2 \\ 4n \cdot \mathbf{\frac{n}{2}} &= 2n^2 \\ 2n^2 &= 2n^2 \\ &\ \square \\\ \\ \end{align*}

• Yes, your proof seems fine to me, but it would be much quicker to use the identity for $n^2$. Sep 28 '17 at 12:52
• @Shaun: You mean $n^2 = 1 + 3 + 5 + ... + 2n-1$? How do you prove that without induction?
– Zaz
Sep 28 '17 at 12:55
• Yes. I don't know, @Zaz; that's a separate question. Sep 28 '17 at 12:56
• Here 's a [Proof without induction][1] of that sum (see answer section). [1]: math.stackexchange.com/questions/1666075/… Sep 28 '17 at 13:04
• Your method is a special case of telescopic induction, e.g. see this answer. Sep 28 '17 at 14:16

The second (and to some extent the first) uses the $\dots$ informal notation. This should probably be seen as a shorthand or more visual way of handling summations. Formally one should have used $\sum$-notation. The problem in the last is that you visually rearranges the terms which is not that good looking when using $\sum$ notation.