A268582 Sphenic numbers having identical digits.

66, 222, 555, 777, 2222, 3333, 5555, 7777, 22222, 33333, 55555, 77777, 2222222, 3333333, 5555555, 7777777, 22222222222, 33333333333, 55555555555, 77777777777, 1111111111111, 22222222222222222, 33333333333333333, 55555555555555555, 77777777777777777, 6666666666666666666

Offset

1,1

Comments

Subsequence of A007304 (sphenic numbers: products of 3 distinct primes).

a(1)= A087331(4).

Links

Max Alekseyev, Table of n, a(n) for n = 1..89

Example

222 is in the sequence because 222 = 2*3*37, product of 3 distinct primes.

Maple

with(numtheory):
for n from 1 to 23 do:
  for b from 1 to 9 do:
    x:=(((10^n)- 1)/9)*b:y:=factorset(x):n1:=nops(y):
     if bigomega(x)=3 and n1=3
      then
      printf(`%d, `, x):
      else
     fi:
   od:
od:

Mathematica

Select[Flatten@ Map[Map[Function[k, FromDigits@ Table[k, {#}]], Range[1, 9]] &, Range@ 20], Length@ # == 3 && Times @@ Last /@ # == 1 &@ FactorInteger@ # &] (* Michael De Vlieger, Feb 07 2016 *)

Crossrefs

Cf. A007304, A046053, A087331, A095370, A196104.

Sequence in context: A074873 A205817 A046393 * A117306 A322768 A158070

Adjacent sequences: A268579 A268580 A268581 * A268583 A268584 A268585

Keyword

nonn,base

Author

Michel Lagneau, Feb 07 2016

Status

approved