login
A056654
Numbers k such that 10*R_k + 3 is prime, where R_k is the repunit (A002275) of length k.
507
0, 1, 2, 4, 8, 10, 23, 83, 220, 1313, 2951, 20015, 51053
OFFSET
1,3
COMMENTS
Also numbers k such that (10^(k+1)+17)/9 is prime.
a(14) > 10^5. - Robert Price, Nov 01 2014
FORMULA
a(n) = A097683(n+1) - 1. - Robert Price, Nov 01 2014
EXAMPLE
8 is a term because 111111113 is a prime.
MATHEMATICA
Do[ m = n; If[ primeQ[ 10*(10^n - 1)/9 + 3 ], Print[ n ] ], {n, 1, 1350} ]
PROG
(PARI) is(n)=ispseudoprime(10^n\9*10+3) \\ Charles R Greathouse IV, Nov 10 2021
CROSSREFS
Cf. A093011 (corresponding primes), A097683.
Sequence in context: A308832 A124849 A376681 * A365002 A370663 A020950
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 09 2000
EXTENSIONS
a(11) (only a probable prime) from Rick L. Shepherd, Mar 14 2004
a(12)-a(13) derived from A097683 by Robert Price, Nov 01 2014
STATUS
approved