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 1-4 of 4 results.

A101758 Semiprime tribonacci numbers.

Original entry on oeis.org

4, 274, 10609, 23837527729, 80641778674, 3122171529233, 408933139743937, 98513851446415969, 7015254043203144209, 1690006574433492223897, 120346786571258131649185, 2533271375725618104752561
Offset: 1

Views

Author

Jonathan Vos Post and Ray Chandler, Dec 26 2004

Keywords

Links

Crossrefs

Cf. A000073, A001358, A101757.

Programs

  • Mathematica
    Select[LinearRecurrence[{1,1,1},{0,1,1},150],PrimeOmega[#]==2&] (* Harvey P. Dale, Oct 16 2012 *)

Formula

a(n) = A000073(A101757(n)).

A363837 Numbers k such that k-th Jacobsthal number A001045(k) is a semiprime.

Original entry on oeis.org

6, 8, 10, 14, 26, 29, 34, 37, 38, 41, 47, 49, 53, 62, 67, 71, 73, 103, 107, 109, 122, 139, 151, 179, 223, 229, 251, 254, 269, 277, 311, 349, 353, 433, 457, 487, 503, 599, 601, 613, 619, 643, 739, 757, 827, 839, 1031, 1061, 1117
Offset: 1

Views

Author

Sean A. Irvine, Oct 19 2023

Keywords

Examples

			10 is a term because Jacobsthal(10) = A001045(10) = 341 = 11*31 is a semiprime.

Links

Crossrefs

Cf. A001045, A001358, A277356 (the actual semiprimes), A250292, A085726, A072381, A101757, A286567, A271314.

Programs

  • PARI
    isok(k) = bigomega((2^k - (-1)^k)/3) == 2; \\ Michel Marcus, Oct 19 2023

Extensions

a(47)-a(49) from Amiram Eldar, Feb 25 2024

A366583 a(2) = a(3) = 1; for n >3, a(n) = smallest prime factor of n-th Tribonacci number.

Original entry on oeis.org

1, 1, 2, 2, 7, 13, 2, 2, 3, 149, 2, 2, 3, 5, 2, 2, 103, 13, 2, 2, 5, 3, 2, 2, 3, 3, 2, 2, 23, 5, 2, 2, 3, 103, 2, 2, 7, 3, 2, 2, 167, 11, 2, 2, 5, 3, 2, 2, 199, 5, 2, 2, 7, 163, 2, 2, 29531, 5, 2, 2, 3, 5, 2, 2, 3, 199, 2, 2, 7, 19, 2, 2, 1259, 3, 2, 2, 3, 3
Offset: 2

Views

Author

Tyler Busby, Oct 13 2023

Keywords

Examples

			For n=24: A000073(24) = 2*2*2*51343, so a(24)=2.

Links

Crossrefs

Cf. A000073, A231575, A101757.

Programs

  • Mathematica
    FactorInteger[#][[1, 1]] & /@ LinearRecurrence[{1, 1, 1}, {1, 1, 2}, 78] (* Amiram Eldar, Oct 23 2023 *)

Formula

a(n) = A020639(A000073(n)).

A366584 a(2) = a(3) = 1; for n >3, a(n) = largest prime factor of n-th Tribonacci number.

Original entry on oeis.org

1, 1, 2, 2, 7, 13, 3, 11, 3, 149, 137, 7, 103, 31, 7, 103, 103, 79, 97, 5501, 3469, 919, 51343, 188869, 853, 1427, 470077, 239, 313, 307, 73, 883483, 11113, 227, 53, 3833, 631, 40093, 4349, 354763, 142739687, 45181, 40320889337, 71584631, 3331, 5500283
Offset: 2

Views

Author

Tyler Busby, Oct 13 2023

Keywords

Examples

			For n=24: A000073(24) = 2*2*2*51343, so a(24)=51343.

Links

Crossrefs

Cf. A000073, A231575, A101757.

Programs

  • Mathematica
    FactorInteger[#][[-1, 1]] & /@ LinearRecurrence[{1, 1, 1}, {1, 1, 2}, 46] (* Amiram Eldar, Oct 23 2023 *)

Formula

a(n) = A006530(A000073(n)).
Showing 1-4 of 4 results.

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

Content is available under The OEIS End-User License Agreement.