login
A120376
Primes of the form 2*5^k - 1.
2
1249, 31249, 305175781249, 119209289550781249, 1862645149230957031249, 111022302462515654042363166809082031249, 25243548967072377773175314089049159349542605923488736152648925781249
OFFSET
1,1
COMMENTS
See comments for A057472. Examined in base 12, all n must be even and all primes must be 1-primes. For example, 1249 is 881 in base 12.
The next term has 125 digits. - Harvey P. Dale, Jan 26 2019
FORMULA
a(n) = 2*5^A120375(n) - 1 = 2*5^(2*A002958(n)) - 1. - Jianing Song, Sep 22 2018
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^k-1; if isprime(n) then printf("%d, %d", k, n) fi od od;
MATHEMATICA
Select[2*5^Range[100]-1, PrimeQ] (* Harvey P. Dale, Jan 26 2019 *)
PROG
(PARI) for(k=1, 1e3, if(ispseudoprime(p=2*5^k-1), print1(p, ", "))); \\ Altug Alkan, Sep 22 2018
CROSSREFS
Integers k such that 2*b^k - 1 is prime: A090748 (b=2), A003307 (b=3), A120375 (b=5), A057472 (b=6), A002959 (b=7), A002957 (b=10), A120378 (b=11).
Primes of the form 2*b^k - 1: A000668 (b=2), A079363 (b=3), this sequence (b=5), A158795 (b=7), A055558 (b=10), A120377 (b=11).
Cf. also A000043, A002958.
Sequence in context: A376502 A086709 A215719 * A231805 A122272 A330650
KEYWORD
nonn
AUTHOR
Walter Kehowski, Jun 28 2006
STATUS
approved