What is the sum of the factors of 15?

Answer 1

Answer

Not sure what your definition of factorial is. But with the usual definition, the sum is,

0!+1!+2!+3!+4! ... = 1+1+2+6+24+.... = Ei(1)/e = .69...

where Ei is the exponential integral function and e is Euler's number. You can compute a few terms to see how it converges :)

To generate the sum of the factorial series nearly as fast the last factorial,

1+1+2+6+24+... = 1+1(1+2(1+3(1+...+14(1+15)...)))

The answer is 120.

Answer 2If interpreted as (1+2+...+15)! the answer would be:
6.6895029134491270575881180540903725867527463331380298... x 10^198

Answer 2

1 + 3 + 5 + 15 = 24