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A121877 Numbers k such that (5^k - 3^k)/2 = A005059(k) is prime. 53
13, 19, 23, 31, 47, 127, 223, 281, 2083, 5281, 7411, 7433, 19051, 27239, 35863, 70327 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

All terms are primes. Their indices are listed in A123704.

Corresponding primes are listed in A123705.

No other terms < 125000. - Robert Price, Aug 15 2011

LINKS

Table of n, a(n) for n=1..16.

FORMULA

a(n) = prime(A123704(n)).

MATHEMATICA

Do[f=(5^n-3^n)/2; If[PrimeQ[f], Print[{n, f}]], {n, 1, 300}]

PROG

(PARI) forprime(p=2, 1e4, if(ispseudoprime((5^p-3^p)>>1), print1(p", "))) \\ Charles R Greathouse IV, Jun 16 2011

CROSSREFS

Cf. A005058, A005059, A109347, A120612, A081186, A121824, A123704, A123705.

Sequence in context: A007627 A180525 A257590 * A109902 A214031 A250293

Adjacent sequences:  A121874 A121875 A121876 * A121878 A121879 A121880

KEYWORD

nonn,hard,more

AUTHOR

Alexander Adamchuk, Aug 31 2006, Oct 08 2006

EXTENSIONS

More terms from Farideh Firoozbakht, Oct 11 2006

a(13)-a(16) from Robert Price, Aug 15 2011

STATUS

approved

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Last modified May 26 15:17 EDT 2019. Contains 323596 sequences. (Running on oeis4.)