Question

What is the sum of all the positive divisors of 72?

Answer 1

$$\sigma(72)=\frac{2^4-1}{2-1}\times\frac{3^3-1}{3-1}=195$$

Answer 2

In general $$\sigma(n)=\prod_{p\mid n}\frac{p^{v_p(n)+1}-1}{p-1}$$

Answer 3

If you want to go "low tech," you can get the positive divisors of 72 and add them.

1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72

Answer 4

Check out the divisors online calculator at:
http://www.javascripter.net/math/calculators/divisorscalculator.htm

Answer 5

thanks