Question

How to determine sum of digit is prime or not?

Answer 1

if the number is not a really order 1 and itself

Answer 2

Take a number N=abcdef...¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯N=abcdef...¯ where a,b,c,d,e,…a,b,c,d,e,… are the digits of NN.

Let kk be the sum of those digits : a+b+c+d+e+...=ka+b+c+d+e+...=k
If kk is any of 1,2,4,5,7,81,2,4,5,7,8 then NN is prime. Otherwise it is not a prime.

Example: N=17N=17 and k=1+7=8,k=1+7=8, Therefore NN is prime.