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 1-3 of 3 results.

A104564 Number of distinct prime divisors of 77...771 (with n 7's).

Original entry on oeis.org

1, 2, 2, 2, 3, 2, 2, 2, 2, 3, 3, 1, 3, 4, 2, 2, 3, 2, 1, 4, 3, 1, 3, 3, 2, 3, 4, 2, 7, 1, 5, 6, 3, 3, 4, 3, 4, 5, 5, 3, 5, 4, 2, 3, 3, 6, 2, 2, 6, 5, 4, 3, 4, 4, 6, 2, 6, 5, 4, 4, 4, 5, 3, 3, 5, 2, 6, 3, 5, 5, 4, 6, 4, 6, 5, 3, 3, 4, 4, 4, 4, 6, 4, 3, 6, 5, 3
Offset: 1

Views

Author

Parthasarathy Nambi, Apr 20 2005

Keywords

Comments

Also number of distinct prime factors of (10^(n + 1) - 1)*7/9 - 6. - Stefan Steinerberger, Mar 01 2006

Examples

			The number of distinct prime divisors of 71 is 1 (prime).
The number of distinct prime divisors of 771 is 2.
The number of distinct prime divisors of 7771 is 2.

Links

Crossrefs

Cf. A001221, A104484, A104483, A173806.
Cf. A104484 (3 instead of 7), A104659 (4 instead of 7), A104517 (5 instead of 7), A104890 (6 instead of 7), A105972 (8 instead of 7), A105259 (9 instead of 7).

Programs

  • Mathematica
    Table[Length[FactorInteger[(10^(n + 1) - 1)*7/9 - 6]], {n, 1, 50}] (* Stefan Steinerberger, Mar 01 2006 *)
PrimeNu/@(FromDigits/@Table[PadLeft[{1},n,7],{n,2,55}])  (* Harvey P. Dale, Apr 22 2011 *)

Formula

a(n) = A001221(A173806(n+1)). - Amiram Eldar, Jan 24 2020

Extensions

More terms from Stefan Steinerberger, Mar 01 2006
Offset corrected and more terms added by Amiram Eldar, Jan 24 2020

A099412 0 together with numbers k such that 4*R_k - 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Original entry on oeis.org

0, 2, 4, 11, 28, 55, 94, 475, 2080, 4835, 5845, 12338, 20194, 49529, 56989, 71546, 77309, 284330
Offset: 1

Views

Author

Robert G. Wilson v, Oct 14 2004

Keywords

Comments

Also numbers k such that abs((4*10^k - 31))/9 is prime.
a(18) > 10^5. - Robert Price Sep 06 2014

Links

Crossrefs

Cf. A002275, A056681, A104659.

Programs

  • Mathematica
    Do[ If[ PrimeQ[ 4(10^n - 1)/9 - 3], Print[n]], {n, 0, 7000}]
  • PARI
    for(n=0,10^4,if(ispseudoprime(abs(4*(10^n-1)/9-3)),print1(n,", "))) \\ Derek Orr, Sep 06 2014

Formula

a(n) = A056681(n-1) + 1.

Extensions

a(12)-a(17) from Kamada data by Robert Price, Sep 06 2014
a(18) from Kamada data by Tyler Busby, Apr 30 2024

A105972 Number of distinct prime divisors of 88...881 (with n 8's).

Original entry on oeis.org

0, 1, 1, 2, 4, 3, 2, 2, 2, 2, 3, 3, 3, 3, 2, 2, 3, 8, 1, 3, 2, 2, 5, 2, 4, 6, 2, 2, 5, 3, 4, 4, 4, 2, 5, 5, 5, 4, 6, 3, 5, 3, 2, 5, 4, 4, 7, 4, 4, 6, 3, 5, 5, 3, 5, 6, 5, 5, 5, 4, 3, 5, 4, 2, 7, 3, 3, 8, 3, 5, 6, 7, 3, 6, 3, 3, 7, 3, 1, 5, 5, 4, 8, 4, 3, 4, 4
Offset: 0

Views

Author

Parthasarathy Nambi, Apr 28 2005

Keywords

Examples

			The number of distinct prime divisors of 81 is 1.
The number of distinct prime divisors of 881 is 1 (prime).
The number of distinct prime divisors of 8881 is 2.

Links

Crossrefs

Cf. A001221, A104564, A104517, A104659, A173810.

Programs

  • Mathematica
    Table[PrimeNu[(8*10^(n+1) - 71)/9], {n,0,50}] (* G. C. Greubel, May 16 2017 *)
PrimeNu/@Table[10 FromDigits[PadRight[{},n,8]]+1,{n,0,90}] (* Harvey P. Dale, Apr 25 2019 *)
  • PARI
    a(n) = omega((8*10^(n+1)-71)/9); \\ Michel Marcus, Jan 27 2014

Formula

a(n) = A001221(A173810(n+1)). - Michel Marcus, Jan 27 2014

Extensions

More terms from Michel Marcus, Jan 27 2014
Showing 1-3 of 3 results.

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

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