OFFSET
1,2
COMMENTS
The density of this sequence is 0.6504456084..., see A273487. - Charles R Greathouse IV, Jul 01 2016
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..10000
Alec Jones, Java program
EXAMPLE
8 is not present in this sequence because 8 = 2^3 and 3 is prime.
96 is not present in this sequence because 96 = 2^5*3^1 and 5 is prime.
MAPLE
a:= proc(n) option remember; local k; for k from
`if`(n=1, 1, a(n-1)+1) while ormap(i->
isprime(i[2]), ifactors(k)[2]) do od; k
end:
seq(a(n), n=1..80); # Alois P. Heinz, Jun 30 2016
MATHEMATICA
lst0={}; Do[lst[n]=Transpose[FactorInteger[n]][[2]]; k=1; While[!(PrimeQ[lst[n][[k]]]||k==Length[lst[n]]), k++]; If[k==Length[lst[n]]&&!PrimeQ[Last[lst[n]]], AppendTo[lst0, n]], {n, 91}]; lst0 (* Waldemar Puszkarz, Jun 09 2016 *)
PROG
(PARI) isok(n)=my(f = factor(n)); for (k=1, #f~, if (isprime(f[k, 2]), return (0)); ); 1; \\ Michel Marcus, Jun 07 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Alec Jones, Jun 07 2016
STATUS
approved