OFFSET
1,2
COMMENTS
a(A185208(n)) = 1. - Reinhard Zumkeller, Nov 01 2012
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
The divisors of 9 are 1,3,9. 1 is one less than 2, a power of a prime. 3 is one less than 4, a power of a prime. And 9 is one less than 10, not a power of a prime. There are therefore 2 such divisors that are each one less than a power of a prime. So a(9)=2.
MATHEMATICA
a[n_] := Select[Divisors[n], PrimeNu[# + 1] == 1 &] // Length; Table[a[n], {n, 1, 105}] (* Jean-François Alcover, Aug 17 2013 *)
Table[DivisorSum[n, 1 &, PrimePowerQ[# + 1] &], {n, 103}] (* Michael De Vlieger, Aug 29 2017 *)
PROG
CROSSREFS
KEYWORD
nonn
AUTHOR
Leroy Quet, Jun 12 2008
EXTENSIONS
Added more terms. - Steven Bi (chenhsi(AT)stanford.edu), Dec 22 2008
Added more terms (Terms 27 - 50). Steven Bi (chenhsi(AT)stanford.edu), Jan 09 2009
Corrected and extended by Ray Chandler, Jun 25 2009
STATUS
approved