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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A217124 Semiprimes whose decimal representation has only digits in {4,5,7}.

Original entry on oeis.org

4, 55, 57, 74, 77, 445, 447, 454, 545, 554, 745, 755, 4474, 4555, 4574, 4577, 4747, 4754, 4757, 4777, 5447, 5455, 5545, 5554, 5747, 5755, 5774, 5777, 7445, 7447, 7454, 7555, 7745, 7747, 7754, 44477, 44554, 44557, 44747, 44755, 45447, 45454, 45455, 45457
Offset: 1

Views

Author

Jonathan Vos Post, Sep 26 2012

Keywords

Comments

Crooked semiprimes. This is to A217048 as integers all of whose numerals are written (san serif) with at least one right or acute angle (A214584) are to numbers using only the curved digits 0, 3, 6, 8 and 9 (A072960). This is to crooked primes (A217039) as semiprimes (A001358) are to primes (A000040).

Examples

			4555 = 5 * 911 is semiprime.

Links

Crossrefs

Cf. A001358, A072960, A214584.

Programs

  • Mathematica
    SemiPrimeQ[n_Integer] := If[Abs[n] < 2, False, (2 == Plus @@ Transpose[FactorInteger[Abs[n]]][[2]])]; Select[Flatten[Table[FromDigits /@ Tuples[{4, 5, 7}, n], {n, 5}]], SemiPrimeQ] (* T. D. Noe, Sep 27 2012 *)
Select[Flatten[Table[FromDigits/@Tuples[{4,5,7},n],{n,5}]],PrimeOmega[ #] == 2&] (* Harvey P. Dale, Sep 21 2016 *)

Formula

A001358 INTERSECTION A214584.

Extensions

Corrected and extended by T. D. Noe, Sep 27 2012

A247016 Triangular numbers A000217 composed of only curved digits {0, 2, 3, 5, 6, 8, 9}.

Original entry on oeis.org

0, 3, 6, 28, 36, 55, 66, 253, 300, 325, 528, 595, 630, 666, 820, 903, 990, 2080, 2556, 2628, 2850, 2926, 3003, 3655, 3828, 5050, 5253, 5356, 5565, 5886, 5995, 6328, 6555, 6903, 8256, 8385, 20503, 22366, 23005, 23220, 23653, 25200, 26335, 26565, 28203, 28680, 28920
Offset: 1

Views

Author

K. D. Bajpai, Sep 09 2014

Keywords

Comments

Intersection of A000217 and A028374.

Examples

			a(10) = 528 is in the sequence because it is A000217(32) and composed of only curved digits 5, 2 and 8.
a(14) = 820 is in the sequence because it is A000217(40) and composed of only curved digits 8, 2 and 0.

Links

Crossrefs

Cf. A000217, A028374, A034470, A072960.

Programs

  • Mathematica
    A247016 = {}; Do[t = n*(n + 1)/2; If[Intersection[IntegerDigits[t], {1, 4, 7}] == {}, AppendTo[A247016, t]], {n,0, 500}]; A247016
Select[Accumulate[Range[0,300]],DigitCount[#,10,1]==DigitCount[#,10,4] == DigitCount[ #,10,7] == 0&] (* Harvey P. Dale, Apr 18 2019 *)
  • Python
    for n in range(2,10**3):
  s = str(int(n*(n-1)/2))
  if not (s.count('1') + s.count('4') + s.count('7')):
    print(int(s),end=', ') # Derek Orr, Sep 18 2014

Extensions

Added starting number 0 (suggested by D. Orr), added A-number in the name and examples. - Wolfdieter Lang, Oct 06 2014

A079657 Fibonacci numbers using only the curved digits 0, 3, 6, 8 and 9.

Original entry on oeis.org

0, 3, 8, 89
Offset: 1

Views

Author

Shyam Sunder Gupta, Jan 23 2003

Keywords

Comments

Next term, if it exists, is > Fibonacci(6000). - Emeric Deutsch, Mar 12 2005
Next term, if it exists, is > Fibonacci(2425000). - Lars Blomberg, May 09 2011

Crossrefs

Cf. A072960.

Programs

  • Maple
    with(combinat): p:=proc(n) if convert(convert(fibonacci(n),base,10),set) subset {0,3,6,8,9} then fibonacci(n) else fi end: seq(p(n),n=0..6000); # Emeric Deutsch, Mar 12 2005

Extensions

Offset changed to 1 by Klaus Brockhaus, May 09 2011
Previous Showing 11-13 of 13 results.

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

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