OFFSET
1,4
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..2000
EXAMPLE
The divisors of 4 are D = {1, 2, 4} and the subsets of D are {{}, {1}, {2}, {4}, {1, 2}, {1, 4}, {2, 4}, {1, 2, 4}}. Taking the product of elements in these subsets and subtracting 1 yields {-1, 0, 1, 3, 1, 3, 7, 7}, of which the primes are {3, 7}.
MATHEMATICA
Do[l = Subsets[Divisors[n]]; l = Union[Map[Times @@ # - 1&, l]]; Print[Length[Select[l, PrimeQ]]], {n, 100}]
Table[Count[Union[Times@@@Subsets[Divisors[n]]]-1, _?PrimeQ], {n, 100}] (* Harvey P. Dale, Mar 31 2018 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Ryan Propper, Jun 04 2006
STATUS
approved