OFFSET
1,1
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
FORMULA
a(n) ~ n. Proof: the density of numbers without a given decimal digit in their prime factors is 0, which can be seen by looking at the first (or second, in the case of 0) digit and removing all primes with that digit. Taken with the 0 density of numbers missing any decimal digit the result is obtained. - Charles R Greathouse IV, May 02 2013
EXAMPLE
1972 = {1,2,7,9} -> 2 * 2 * 17 * 29, so 1972 is a term.
MATHEMATICA
Fac[n_]:=Sort[DeleteDuplicates[Flatten[IntegerDigits[Take[FactorInteger[n], All, 1]]]]]; Fn[n_]:=Sort[DeleteDuplicates[IntegerDigits[n]]]; t={}; Do[If[! PrimeQ[n]&&Fac[n]===Fn[n], AppendTo[t, n]], {n, 2, 15100}]; t (* Jayanta Basu, May 02 2013 *)
PROG
(PARI) is(n)=if(isprime(n)||n<9, return(0)); my(f=factor(n)[, 1], v=[]); for(i=1, #f, v=concat(v, digits(f[i]))); vecsort(digits(n), , 8)==vecsort(v, , 8) \\ Charles R Greathouse IV, May 02 2013
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Patrick De Geest, Nov 15 1998
EXTENSIONS
Definition corrected by Charles R Greathouse IV, May 02 2013
STATUS
approved