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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A096506 Numbers n for which 2*R_n + 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n.

Original entry on oeis.org

1, 2, 3, 8, 11, 36, 95, 101, 128, 260, 351, 467, 645, 1011, 1178, 1217, 2442, 3761, 3806, 15617, 26459, 63117, 88545, 93497
Offset: 1

Views

Author

Labos Elemer, Jul 12 2004

Keywords

Comments

Also numbers n such that (2*10^n + 7)/9 is prime.
Per Kamada link, 181457, 202059, 262874 are also terms, found by Rytis Slatkevicius. - Michael S. Branicky, Sep 13 2024

Examples

			n=36: 222222222222222222222222222222222223 is a prime number.

Links

Crossrefs

Cf. A056656, A093162, A096503, A096504, A096505, A096507, A096508.

Programs

  • Mathematica
    Do[ If[ PrimeQ[ 2(10^n - 1)/9 + 1], Print[n]], {n, 7000}] (* Robert G. Wilson v, Oct 14 2004 *)

Formula

a(n) = A056656(n) + 1.

Extensions

a(20)-a(24) from Kamada link by Ray Chandler, Feb 27 2012

A093162 Primes of the form 20*R_k + 3, where R_k is the repunit (A002275) of length k.

Original entry on oeis.org

3, 23, 223, 22222223, 22222222223, 222222222222222222222222222222222223
Offset: 1

Views

Author

Rick L. Shepherd, Mar 26 2004

Keywords

Comments

The next term (a(7)) has 95 digits. - Harvey P. Dale, Dec 26 2022

Links

Crossrefs

Cf. A002275, A056656 (corresponding k, and count of digits 2 in a(n)), A096506.

Programs

  • Mathematica
    Select[Table[FromDigits[PadLeft[{3},n,2]],{n,40}],PrimeQ] (* Harvey P. Dale, Dec 26 2022 *)

Formula

a(n) = (20*10^A056656(n) + 7)/9 = (2*10^A096506(n) + 7)/9.

Extensions

Edited by Ray Chandler, Feb 27 2012
Showing 1-2 of 2 results.

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

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