The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

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