

A120375


Integers k such that 2*5^k  1 is prime.


5



4, 6, 16, 24, 30, 54, 96, 178, 274, 1332, 2766, 3060, 4204, 17736, 190062, 223536, 260400, 683080
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OFFSET

1,1


COMMENTS

See comments for A057472. Examined in base 12, all n must be even and all primes must be 1primes. For example, 1249 is 881 in base 12.


LINKS



FORMULA



EXAMPLE

a(1) = 4 since 2*5^4  1 = 1249 is the first prime.


MAPLE

for w to 1 do for k from 1 to 2000 do n:=2*5^k1; if isprime(n) then printf("%d, %d ", k, n) fi od od;


MATHEMATICA

Select[Range[0, 100], PrimeQ[2*5^#  1] &] (* Robert Price, Mar 14 2015 *)


PROG

(PARI) isok(k) = ispseudoprime(2*5^k1); \\ Altug Alkan, Sep 22 2018


CROSSREFS



KEYWORD

nonn,more


AUTHOR



EXTENSIONS

a(14) from Herman Jamke (hermanjamke(AT)fastmail.fm), May 02 2007


STATUS

approved



