OFFSET
1,3
COMMENTS
a(9)=0. Proof: Both p-1 and p+1 are even and composite hence 9=1+8 and 9=2+7 are ruled out, the only possibilities that remain are 9 = 3+6, or 9=4+5. 3+6 is ruled out as 4 is the only even number with 3 divisors. 4+5 is ruled out as 16 is the only even number with 5 divisors.
a(15) = a(19) = 0 is also provable. - David Wasserman, Nov 17 2005
FORMULA
Least prime p such that A175144(p) = n.
EXAMPLE
a(10) = 11, tau(10) = 4 and tau(12) = 6, 4+6=10.
a(16) = 41, a(17) = 101.
MATHEMATICA
nn = 60; t = Table[-1, {nn}]; t[[{1, 2, 4, 6, 9, 15, 19}]] = 0; cnt = 7; p = 1; While[cnt < nn, p = NextPrime[p]; s = DivisorSigma[0, p-1] + DivisorSigma[0, p+1]; If[s <= nn && t[[s]] == -1, t[[s]] = p; cnt++]]; t (* T. D. Noe, Apr 28 2011 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Amarnath Murthy, Dec 02 2003
EXTENSIONS
More terms from David Wasserman, Nov 17 2005
STATUS
approved