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A115987 Numbers m such that 3^m is the average of two primes p and nextprime(p). 3
2, 4, 18, 20, 71, 80, 216, 496, 546, 817, 985, 1341, 1412 (list; graph; refs; listen; history; text; internal format)
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 A220586

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

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Last modified September 29 06:49 EDT 2022. Contains 357082 sequences. (Running on oeis4.)