cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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A056655 Numbers k such that 10*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Original entry on oeis.org

0, 1, 3, 4, 7, 22, 28, 39, 130, 135, 214, 610, 766, 2152, 2575, 22972, 42688, 85711, 85863, 112066, 538507, 631714
Offset: 1

Views

Author

Robert G. Wilson v, Aug 09 2000

Keywords

Comments

Also numbers k such that (10^(k+1)+53)/9 is prime.
2575 also produces a probable prime.
a(20) > 10^5. - Robert Price, Jan 13 2015
a(23) > 670000 (per the Kamada link). - Bill McEachen, Mar 02 2024

Links

Crossrefs

Cf. A093139 (corresponding primes), A097684.

Programs

  • Mathematica
    Do[ If[ PrimeQ[ 10*(10^n - 1)/9 + 7 ], Print[ n ] ], {n, 0, 1250} ]

Formula

a(n) = A097684(n) - 1 for all n >= 0. - Rick L. Shepherd, Aug 23 2004

Extensions

a(14) (giving a probable prime) from Rick L. Shepherd, Mar 23 2004
a(15) from Rick L. Shepherd, Aug 23 2004
a(16)-a(19) derived from A097684 by Robert Price, Jan 13 2015
a(20)-a(22) from the Kamada link by Bill McEachen, Mar 02 2024
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Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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