

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^n3^n)/2; If[PrimeQ[f], Print[{n, f}]], {n, 1, 300}]


PROG

(PARI) forprime(p=2, 1e4, if(ispseudoprime((5^p3^p)>>1), print1(p", "))) \\ Charles R Greathouse IV, Jun 16 2011


CROSSREFS

Cf. A005058, A005059, A109347, A120612, A081186, A121824, A123704, A123705.
Sequence in context: A180525 A335806 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



