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.

Showing 1-2 of 2 results.

A056664 Numbers k such that 80*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Original entry on oeis.org

2, 18, 78, 138, 222, 462, 543, 1095, 1418, 3246, 3876, 4416, 9506, 11090, 14601, 27810, 29187
Offset: 1

Views

Author

Robert G. Wilson v, Aug 09 2000

Keywords

Comments

Also numbers k such that (8*10^(k+1) - 71)/9 is prime.
There are no other terms <= 2500. - Rick L. Shepherd, Mar 02 2004
a(18) > 10^5. - Robert Price, Nov 01 2014

Links

Crossrefs

Cf. A002275, A092675 (corresponding primes), A099421.

Programs

  • Mathematica
    Do[ If[ PrimeQ[ 80*(10^n - 1)/9 + 1 ], Print[n]], {n, 15000}]

Formula

a(n) = A099421(n+1) - 1. - Robert Price, Nov 01 2014

Extensions

a(9) (giving a probable prime) from Rick L. Shepherd, Mar 02 2004
a(10)-a(15) from N. J. A. Sloane, Feb 20 2005
a(16)-a(17) derived from A099421 by Robert Price, Nov 01 2014

A173810 a(n) = (8*10^n - 71)/9 for n > 0.

Original entry on oeis.org

1, 81, 881, 8881, 88881, 888881, 8888881, 88888881, 888888881, 8888888881, 88888888881, 888888888881, 8888888888881, 88888888888881, 888888888888881, 8888888888888881, 88888888888888881, 888888888888888881, 8888888888888888881, 88888888888888888881, 888888888888888888881
Offset: 1

Views

Author

Vincenzo Librandi, Feb 25 2010

Keywords

Links

Crossrefs

Cf. A092675.

Programs

Formula

a(n) = 10*a(n-1) + 71 for n > 0, a(0) = -7.
From Vincenzo Librandi, Jul 05 2012: (Start)
G.f.: x*(1+70*x)/((1-x)*(1-10*x)).
a(n) = 11*a(n-1) - 10*a(n-2). (End)
E.g.f.: exp(x)*(8*exp(9*x) - 71)/9. - Elmo R. Oliveira, Sep 09 2024
Showing 1-2 of 2 results.

Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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