login
A268582
Sphenic numbers having identical digits.
1
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
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
KEYWORD
nonn,base
AUTHOR
Michel Lagneau, Feb 07 2016
STATUS
approved