A115987 Numbers m such that 3^m is the average of two primes p and nextprime(p).

2, 4, 18, 20, 71, 80, 216, 496, 546, 817, 985, 1341, 1412

Offset

1,1

Comments

a(14) > 4000. - Donovan Johnson, Jan 26 2011

Links

Table of n, a(n) for n=1..13.

Example

Nextprime(387420479) = 387420499 and (387420479 + 387420499)/2 = 387420489 = 3^18, so 18 belongs to the sequence.

Mathematica

okQ[n_] := Module[{x = 3^n}, (NextPrime[x, -1] + NextPrime[x])/2 == x]; Select[Range[500], okQ] (* Harvey P. Dale, Jan 19 2011 *)

Prog

(PARI) isok(m) = precprime(3^m-1)+nextprime(3^m+1)==2*3^m; \\ Jinyuan Wang, Feb 26 2020

Crossrefs

Cf. A115986, A115988, A115989.

Sequence in context: A137933 A143116 A304856 * A076692 A350629 A357998

Adjacent sequences: A115984 A115985 A115986 * A115988 A115989 A115990

Keyword

nonn,more

Author

Giovanni Resta, Feb 10 2006

Extensions

a(8) from Harvey P. Dale, Jan 19 2011

a(9)-a(13) from Donovan Johnson, Jan 26 2011

Status

approved