Description: Define the sum of positive divisors function ( x sigma n ) , which
is the sum of the xth powers of the positive integer divisors of n, see
definition in ApostolNT p. 38. For x = 0 , ( x sigma n )
counts the number of divisors of n , i.e. ( 0 sigma n ) isthe
divisor function, see remark in ApostolNT p. 38. (Contributed by Mario Carneiro, 22-Sep-2014)