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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
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified December 6 03:18 EST 2016. Contains 278772 sequences.