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 A076806 Minimal odd k such that k*2^n-1 and k*2^n+1 are twin primes. 2
 3, 1, 9, 15, 81, 3, 9, 57, 45, 15, 99, 165, 369, 45, 345, 117, 381, 3, 69, 447, 81, 33, 1179, 243, 765, 375, 81, 387, 45, 345, 681, 585, 375, 267, 741, 213, 429, 3093, 165, 267, 255, 1095, 9, 147, 849, 405, 1491, 177, 1941, 927, 1125, 1197, 2001, 333, 519 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS A. V. Kulsha, More terms Author?, More information EXAMPLE a(4)=15 because k*2^4-1 and k*2^4+1 are twin primes for k=15 and are not twin primes for smaller odd k. MATHEMATICA f[n_] := Block[{k = 1}, While[ !PrimeQ[k*2^n - 1] || !PrimeQ[k*2^n + 1], k += 2]; k]; Array[f, 50] PROG (PARI) for(n=1, 100, N=2^n; forstep(k=1, 10^100, 2, if(isprime(k*N-1) && isprime(k*N+1), print1(k, ", "); break))) (Sage) A076806 = lambda n: next(k for k in IntegerRange(1, infinity, 2) if is_prime(k*2**n-1) and is_prime(k*2**n+1)) [D. S. McNeil, Dec 8 2010] CROSSREFS Sequence in context: A162749 A094796 A056843 * A111568 A209324 A121489 Adjacent sequences:  A076803 A076804 A076805 * A076807 A076808 A076809 KEYWORD nonn AUTHOR Andrey V. Kulsha, Nov 18 2002 STATUS approved

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