OFFSET
1,1
COMMENTS
a(14) > 4000. - Donovan Johnson, Jan 26 2011
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
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