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 A112545 Least odd number k greater than 1 such that the sum of the predecessor and successor primes of the n-th prime is divisible by k or if no such odd k exists then 2. 2
 7, 5, 2, 5, 7, 2, 5, 3, 3, 3, 3, 5, 11, 3, 53, 3, 3, 3, 5, 3, 3, 3, 3, 5, 5, 13, 53, 5, 59, 61, 3, 3, 11, 5, 3, 157, 3, 3, 173, 3, 5, 11, 97, 7, 3, 211, 3, 113, 5, 3, 3, 5, 3, 257, 263, 3, 3, 3, 5, 7, 5, 151, 5, 157, 7, 3, 3, 7, 5, 3, 3, 3, 373, 3, 3, 3, 5, 13, 5, 5, 5, 7, 3, 3, 3, 3, 5, 5, 29, 3, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,1 COMMENTS From Robert Israel, Apr 20 2017: (Start) a(n) = A078701(prime(n-1)+prime(n+1)) unless that is 1, in which case a(n)=2. a(n) = 2 if and only if for some m, A007053(m) = n or n-1 with prime(n-1)+prime(n+1) = 2^(m+1). The first m for which this occurs are 3,4,9,379,593, corresponding to n = 4,7,97 and approximately 3*10^116 and 1*10^181.  Are there infinitely many? (End) LINKS Robert Israel, Table of n, a(n) for n = 2..10000 MAPLE f:= proc(n) local t; t:= min(numtheory:-factorset(ithprime(n-1)+ithprime(n+1)) minus {2}); if t::integer then t else 2 fi end proc: map(f, [\$2..200]); # Robert Israel, Apr 20 2017 MATHEMATICA f[n_] := Block[{k = 3, s = Prime[n - 1] + Prime[n + 1]}, While[Mod[s, k] != 0 && k <= s, k += 2]; If[k > s, 2, k]]; Table[ f[n], {n, 2, 92}] PROG (PARI) a(n) = {p = prime(n); s = precprime(p-1) + nextprime(p+1); f = factor(s); if (#f~ > 1, f[2, 1], f[1, 1]); } \\ Michel Marcus, Apr 22 2017 CROSSREFS Cf. A000040, A112686, A112681, A112794, A112731, A112789, A112795, A112796, A112804, A112847, A112859, A113155, A113156, A113157, A113158. Cf. A007053, A078701. Sequence in context: A258370 A135537 A212038 * A021934 A021097 A307160 Adjacent sequences:  A112542 A112543 A112544 * A112546 A112547 A112548 KEYWORD nonn AUTHOR Robert G. Wilson v, Jan 11 2006 STATUS approved

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Last modified December 10 00:54 EST 2019. Contains 329885 sequences. (Running on oeis4.)