OFFSET
3,1
COMMENTS
LINKS
Gokberk Yaltirakli, Table of n, a(n) for n = 3..1024 (terms 3..100 from Harvey P. Dale)
MATHEMATICA
lsp[n_]:=Module[{sp=NextPrime[2^n, -1]}, While[!PrimeQ[(sp-1)/2], sp= NextPrime[ sp, -1]]; sp]; Array[lsp, 35, 3] (* Harvey P. Dale, Feb 10 2019 *)
PROG
(MIT/GNU Scheme, with Aubrey Jaffer's SLIB Scheme library)
(require 'factor) ;; For prime? predicate.
(define (A243916 n) (if (< n 3) 0 (let loop ((i (- (expt 2 n) 1))) (cond ((and (prime? i) (prime? (/ (- i 1) 2))) i) (else (loop (- i 2)))))))
;; Antti Karttunen, Jun 18 2014
(Python)
from sympy import isprime
def a(n):
if n<3: return 0
i=2**n - 1
while True:
if isprime(i) and isprime((i - 1)/2): return i
else: i-=2 # Indranil Ghosh, Jun 12 2017, after Antti Karttunen's Scheme Code
CROSSREFS
KEYWORD
nonn
AUTHOR
José Eduardo Gaboardi de Carvalho, Jun 18 2014
STATUS
approved