Trying to read Simple Wikipedia's Mathematical Induction article:


In many of the images, like this one:

There is a 2 to the right sorry, left of the summation symbol. What does it mean?

  • $\begingroup$ It's just a multiplicative factor. You should read it as "2 times the sum of...". $\endgroup$ – Hubble Oct 3 '16 at 19:00
  • $\begingroup$ 2 * the sum of all of those numbers leading up to n (aka, 2 * n)? It's not really clicking with me... $\endgroup$ – user156964 Oct 3 '16 at 19:07
  • $\begingroup$ $2 \times \left(\sum_{k=1}^n k\right) = n(n+1)$. $\endgroup$ – Hubble Oct 3 '16 at 19:11
  • $\begingroup$ Ok, so the idea is that you keep adding up until what you're adding is the number n. And that first image I linked is rewritten from wikimedia.org/api/rest_v1/media/math/render/svg/… with the 1/2 taken from the left and added as a times 2 on the right. Thanks. $\endgroup$ – user156964 Oct 3 '16 at 19:18

If you have $a\sum_{b=1}^{c}d$, that means you multiply $a$ by $\sum_{b=1}^{c}d$. Thus, you take $2$ and multiply it by $\sum_{k=1}^{n}k$.

  • $\begingroup$ I believe it does. I told user156964 what a number to the right of a summation means. $\endgroup$ – 4yl1n Mar 25 '20 at 19:34
  • $\begingroup$ You are right. OP edited to ask what a number to left means. Your answer is useful - I upvoted. $\endgroup$ – G. Chiusole Mar 25 '20 at 19:38

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