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A120376 Primes of the form 2*5^k - 1. 2
1249, 31249, 305175781249, 119209289550781249, 1862645149230957031249, 111022302462515654042363166809082031249, 25243548967072377773175314089049159349542605923488736152648925781249 (list; graph; refs; listen; history; text; internal format)
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

LINKS

Table of n, a(n) for n=1..7.

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: A020384 A086709 A215719 * A231805 A122272 A330650

Adjacent sequences:  A120373 A120374 A120375 * A120377 A120378 A120379

KEYWORD

nonn

AUTHOR

Walter Kehowski, Jun 28 2006

STATUS

approved

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Last modified May 18 10:19 EDT 2021. Contains 343995 sequences. (Running on oeis4.)