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.

User: Zachary M Franco

Zachary M Franco's wiki page.

Zachary M Franco has authored 3 sequences.

A350211 Numbers k such that the arithmetic mean of the digits of k! is an integer.

Original entry on oeis.org

0, 1, 2, 3, 4, 5, 6, 12, 26, 28, 32, 59, 262, 391, 533, 579
Offset: 1

Author

Zachary M Franco, Dec 19 2021

Keywords

Comments

A heuristic argument suggests that this short list is complete. By Stirling's approximation, n! has order n*log(n) digits of which n/4 are terminal zeros. If the remaining digits are random, the mean will be just below 4.5. For n > 6, n! and also its digits sum are divisible by 9. 12! is the only factorial with 9 digits. The others have 27, 30, 36, 81, 522, 846, 1224, and 1350 digits, respectively.

Examples

			4 is a term because 4! = 24 and (2+4)/2 = 3 is an integer.
		

Crossrefs

Programs

  • Maple
    q:= n-> (f-> (add(i, i=convert(f, base, 10))/length(f))::integer)(n!):
    select(q, [$0..1000])[];  # Alois P. Heinz, Dec 19 2021
  • Mathematica
    Do[If[IntegerQ[Mean[IntegerDigits[n!]]], Print[n, " ", Mean[IntegerDigits[n!]]]], {n, 1, 100000}]
  • PARI
    isok(k) = my(d=digits(k!)); (vecsum(d) % #d) == 0; \\ Michel Marcus, Dec 19 2021

A322158 a(n) is the smallest m for which binomial(m,5) has exactly n distinct prime factors.

Original entry on oeis.org

6, 9, 11, 22, 25, 70, 78, 276, 497, 990, 1771, 8178, 20504, 44254, 181051, 416328, 1013728, 3383579, 8667726, 34332376, 122289552, 244215150, 969751302, 1865174676, 6648863728, 26888317326, 107132035803
Offset: 2

Author

Zachary M Franco, Nov 27 2018

Keywords

Comments

Binomial(m,5) is never prime, so the offset is 2.

Crossrefs

Programs

  • Mathematica
    a[n_] := Module[{m = 5}, While[PrimeNu[Binomial[m, 5]] != n, m++]; m]; Array[a, 10, 2] (* Amiram Eldar, Nov 29 2018 *)
  • PARI
    a(n) = for(m=5, oo, if(omega(binomial(m, 5))==n, return(m))) \\ Felix Fröhlich, Dec 01 2018

Extensions

a(22)-a(23) from Chai Wah Wu, Dec 29 2018
a(24)-a(28) from Giovanni Resta, Jan 04 2019

A321852 a(n) is the smallest m for which binomial(m, 6) has exactly n distinct prime factors.

Original entry on oeis.org

6, 7, 8, 9, 10, 18, 26, 40, 77, 120, 210, 477, 715, 2227, 3290, 9065, 17020, 49915, 139195, 240465, 721929, 1124840, 4455445, 16319578, 26683220, 105655905, 134879176, 677868170, 3290262264
Offset: 0

Author

Zachary M Franco, Nov 19 2018

Keywords

Crossrefs

Programs

  • Mathematica
    a[n_] := Module[{m=6, t=1}, While[PrimeNu[t] != n, m++; t*=m/(m-6)]; m]; Array[a, 20] (* Amiram Eldar, Nov 27 2018 *)
  • PARI
    a(n)={my(m=6, t=1); while(omega(t)<>n, m++; t*=m/(m-6)); m} \\ Andrew Howroyd, Nov 26 2018

Extensions

a(22)-a(28) from Giovanni Resta, Nov 27 2018
a(0) prepended by Jianing Song, Dec 31 2018