# How are formulas with inductive proofs discovered?

## Main Question or Discussion Point

There are certain formulas for which only inductive proofs are known. But since we need to know the formula first to prove it using mathematical induction, how do they get the formula in the first place?
Here is an example: - Euler's formula in graph theory states that v-e+f=2 for all planar graphs. I think only an inductive proof is currently known. So how did he come up with this formula without proving it first?...guessing?

Here is some interesting information (courtesy of micromass):-

www.homepages.math.uic.edu/~kauffman/DCalc.pdf [Broken]

I believe the topic of generatingfunctionology answers yours question, though I'm no expert.

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There are different ways.
1. Try to find a formula empirically. For example, if you want to know a formula for the nth triangular number 1+2+3+...+n, you could compute the first 20 and try to find a relationship with n by examining the numbers. You can notice a lot of patterns just be examining data closely.

2. Sometimes you can know a lot about a problem without completely understanding it. In those cases you might have reason to believe something but not be able to prove it. For example, you might suspect that ln(n) - 1 -1/2 -1/3 - ... - 1/n approaches a limit without being able to prove it. Basically, even if you don't understand something very well, you can try to apply induction. You can "get lucky" with induction by proving something that you don't understand that well. The drawback is that such proofs don't always add much to your understanding or suggest new avenues to pursue.

I suspect that empirical observation is the main way that people conjecture formulas before they understand them.

chiro