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.

Previous Showing 21-25 of 25 results.

A036322 Composite numbers whose prime factors contain no digits other than 6 and 7.

Original entry on oeis.org

49, 343, 469, 2401, 3283, 4489, 4739, 16807, 22981, 31423, 33173, 45359, 117649, 160867, 219961, 232211, 300763, 317513, 458329, 474439, 536669, 537439, 823543, 1126069, 1539727, 1625477, 2105341, 2222591, 3039053, 3208303, 3321073
Offset: 1

Views

Author

Patrick De Geest, Dec 15 1998

Keywords

Comments

The multiplicative closure of A020469 consists of itself, 1, and this sequence. - Charles R Greathouse IV, Jun 25 2014

Links

Crossrefs

Cf. A020469, A036302-A036325.

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020469} (p/(p - 1)) - Sum_{p in A020469} 1/p - 1 = 0.0268448443... . - Amiram Eldar, May 22 2022

A036323 Composite numbers whose prime factors contain no digits other than 7 and 8.

Original entry on oeis.org

49, 343, 2401, 5509, 6139, 6209, 16807, 38563, 42973, 43463, 55139, 62209, 117649, 269941, 300811, 304241, 385973, 435463, 551509, 552139, 552209, 615139, 615209, 619369, 690199, 698069, 769129, 777899, 786769, 823543, 1889587, 2105677
Offset: 1

Views

Author

Patrick De Geest, Dec 15 1998

Keywords

Comments

All terms are a product of at least two terms of A020470. - David A. Corneth, Oct 09 2020

Links

Crossrefs

Cf. A020470, A036302-A036325.

Programs

  • Mathematica
    Select[Range[25*10^5],CompositeQ[#]&&SubsetQ[{7,8},Flatten[ IntegerDigits/@ Transpose[ FactorInteger[#]][[1]]]]&] (* Harvey P. Dale, Jan 19 2015 *)

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020470} (p/(p - 1)) - Sum_{p in A020470} 1/p - 1 = 0.0244618735... . - Amiram Eldar, May 22 2022

A036951 Smallest n-digit prime containing only the digits 8 and 9, or 0 if no such prime exists.

Original entry on oeis.org

0, 89, 0, 8999, 89899, 888989, 8888989, 88888999, 888898889, 8888888989, 88888888999, 888888898999, 8888888999899, 88888888888889, 888888888898999, 8888888888989999, 88888888888888889, 888888888888898889
Offset: 1

Views

Author

Patrick De Geest, Jan 04 1999

Keywords

Links

Crossrefs

Cf. A020472, A036229, A036325.

Programs

  • Mathematica
    Table[SelectFirst[10#+9&/@FromDigits/@Tuples[{8,9},n-1],PrimeQ],{n,20}]/. (Missing["NotFound"]->0) (* Harvey P. Dale, Feb 01 2018 *)

A036311 Composite numbers whose prime factors contain no digits other than 2 and 5.

Original entry on oeis.org

4, 8, 10, 16, 20, 25, 32, 40, 50, 64, 80, 100, 125, 128, 160, 200, 250, 256, 320, 400, 500, 512, 625, 640, 800, 1000, 1024, 1250, 1280, 1600, 2000, 2048, 2500, 2560, 3125, 3200, 4000, 4096, 5000, 5120, 6250, 6400, 8000, 8192, 10000, 10240, 12500, 12800
Offset: 1

Views

Author

Patrick De Geest, Dec 15 1998

Keywords

Comments

A003592 with 1, 2 and 5 removed. - Robert Israel, Apr 29 2018

Links

Crossrefs

Cf. A003592, A036302-A036325, A025612.

Programs

  • Magma
    [n: n in [4..13000] | not IsPrime(n) and forall{f: f in PrimeDivisors(n) | Intseq(f) subset [2,5]}]; // Bruno Berselli, Aug 26 2013
  • Maple
    N:= 20000: # to get all terms <= N
S:= {seq(seq(2^i*5^j,i=0..ilog2(N/5^j)),j=0..floor(log[5](N)))} minus {1,2,5}:
sort(convert(S,list)); # Robert Israel, Apr 29 2018
  • Mathematica
    dpfQ[n_]:=Module[{d=Union[Flatten[IntegerDigits/@Transpose[FactorInteger[n]][[1]]]]}, !PrimeQ[n]&&(d == {2}||d == {5}||d == {2, 5})]; Select[Range[15000], dpfQ] (* Vincenzo Librandi, Aug 25 2013 *)

Formula

Sum_{n>=1} 1/a(n) = 4/5. - Amiram Eldar, May 18 2022~

A036928 Composite numbers whose prime factors contain no digits other than 0 and 1.

Original entry on oeis.org

121, 1111, 1331, 10201, 12221, 14641, 111221, 112211, 134431, 161051, 1021211, 1030301, 1112221, 1223431, 1234321, 1478741, 1771561, 10212211, 11121011, 11233321, 11333311, 12101111, 12234431, 13457741, 13577531, 16266151
Offset: 1

Views

Author

Patrick De Geest, Jan 04 1999

Keywords

Links

Crossrefs

Cf. A020449, A036302-A036325, A036929.

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020449} (p/(p - 1)) - Sum_{p in A020449} 1/p - 1 = 0.0102023428... . - Amiram Eldar, May 18 2022

Extensions

Description clarified by Ray Chandler, Nov 07 2008
Previous Showing 21-25 of 25 results.

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

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