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.

A092839 Indices of primes in A001003.

Original entry on oeis.org

3, 4, 6, 10, 216
Offset: 1

Views

Author

Eric W. Weisstein, Mar 07 2004

Keywords

Comments

a(6), if it exists, is > 1.5*10^6. - Robert Price, Apr 16 2014

Links

Crossrefs

Cf. A001003, A092840.

Programs

  • PARI
    s(m)= { if (m==1, return(a1)); if (m==2, return(a2)); r = (3*(2*m - 3)*a2 - (m - 3)*a1)/m; a1=a2; a2=r; return(r); }
{ a1=1; a2=1; n=0; for (m=1, 300, a=s(m); if (isprime(a), n++; print1(m, ", "))); } \\ Harry J. Smith, Jun 21 2009

A101618 Indices of semiprimes in A001003.

Original entry on oeis.org

7, 11, 17, 19, 21, 239
Offset: 1

Views

Author

Jonathan Vos Post, Dec 09 2004

Keywords

Comments

No more terms < 2000. - David Wasserman, Mar 27 2008
a(7) >= 4377. A001003(4377) is a 3345-digit composite number with unknown factorization. Other possible terms are 4391, 45207, 45369, 45377, 131723, 131733, ... - Tyler Busby, Feb 08 2023

Examples

			a(1) = 7 because A001003(7) = 4279 = 11 * 389.
a(2) = 11 because A001003(11) = 2646723 = 3 * 882241.
a(3) = 17 because A001003(17) = 55909013009 = 41263 * 1354943.
a(4) = 19 because A001003(19) = 1618362158587 = 24413 * 66290999.
a(5) = 21 because A001003(21) = 47574827600981 = 3253 * 14624908577.

Links

  • Eric Weisstein's World of Mathematics, Semiprime.

Crossrefs

Cf. A001003, A001358.
Cf. A101619, A092839, A092840.

Formula

A001003(a(n)) is in A001358 (semiprimes).

Extensions

Edited by Franklin T. Adams-Watters, Nov 07 2006
Changed by Michael Somos, Mar 31 2007, because of the re-indexing of A001003.
a(6) from David Wasserman, Mar 27 2008

A101619 Semiprimes in A001003.

Original entry on oeis.org

4279, 2646723, 55909013009, 1618362158587, 47574827600981
Offset: 1

Views

Author

Jonathan Vos Post, Dec 09 2004

Keywords

Comments

Next term has 180 digits and is too large to include. - David Wasserman, Mar 27 2008

Links

  • Eric Weisstein's World of Mathematics, Semiprime.

Crossrefs

Cf. A001003, A001358.
Cf. A101618, A092839, A092840.

Formula

a(n) = A001003(A101618(n)). - David Wasserman, Mar 27 2008

Extensions

Edited by Franklin T. Adams-Watters, Nov 07 2006

A212276 Prime generalized Catalan numbers.

Original entry on oeis.org

2, 17, 37, 15101701
Offset: 1

Views

Author

Jonathan Vos Post, May 13 2012

Keywords

Comments

A004148(n) for n = 3, 6, 7, 22, no more through 70. Note that Koshy and Salmassi give an elementary proof that the only prime Catalan numbers A000108 are a(2) = 2 and a(3) = 5.
The next term, if it exists, has more than 2000 digits. - Charles R Greathouse IV, May 13 2012

Crossrefs

Cf. A000040, A000108, A004148, A092840.

Formula

A004148 INTERSECTION A000040.
Showing 1-4 of 4 results.

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

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