A189536 The smallest prime p such that tau(p-1) + tau(p+1) = prime(n), or 0 if no such prime exists; where tau(k) is the number of divisors of k.

0, 2, 3, 5, 17, 37, 101, 0, 401, 3137, 4357, 62501, 21317, 16901, 1008017, 15877, 1020101, 33857, 69697, 14401, 331777, 78401, 32401, 57601, 828101

Offset

1,2

Comments

This is sequence A090482(n) for prime n.

Links

Table of n, a(n) for n=1..25.

Mathematica

nn = 25; t = Table[-1, {nn}]; t[[1]] = 0; t[[8]] = 0; cnt = 2; p = 1; While[cnt < nn, p = NextPrime[p]; s = DivisorSigma[0, p - 1] + DivisorSigma[0, p + 1]; If[PrimeQ[s], i = PrimePi[s]; If[i <= nn && t[[i]] == -1, t[[i]] = p; cnt++]]]; t (* T. D. Noe, Apr 28 2011 *)

Crossrefs

Cf. A000005, A000668, A002496, A090482, A175144 (tau(p-1)+tau(p+1)).

Sequence in context: A077498 A118958 A259596 * A163588 A270539 A053182

Adjacent sequences: A189533 A189534 A189535 * A189537 A189538 A189539

Keyword

nonn

Author

Juri-Stepan Gerasimov, Apr 23 2011

Status

approved