OFFSET
1,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
a(1)=1 because the 1st composite number is 4, and the prime factors of 4 are (2,2): (2-1)*(2-1)=1.
a(4)=4 because the 4th composite number is 9, and the prime factors of 9 are (3,3): (3-1)*(3-1)=4.
a(19)=8 because the 19th composite number is 30, and the prime factors of 30 are (2,3,5): (2-1)*(3-1)*(5-1)=8.
MAPLE
b:= proc(n) option remember; local k;
for k from 1+`if`(n=1, 3, b(n-1))
while isprime(k) do od; k
end:
a:= n-> mul((i[1]-1)^i[2], i=ifactors(b(n))[2]):
seq(a(n), n=1..100); # Alois P. Heinz, Oct 23 2014
MATHEMATICA
b[n_] := Product[{p, e} = pe; (p-1)^e, {pe, FactorInteger[n]}];
b /@ Select[Range[100], CompositeQ] (* Jean-François Alcover, Nov 13 2020 *)
PROG
(PARI) b(n) = my(f=factor(n)); f[, 1] = apply(x->(x-1), f[, 1]); factorback(f); \\ A003958
lista(nn) = apply(b, select(x->((x != 1) && !isprime(x)), [1..nn])); \\ Michel Marcus, Nov 13 2020
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Gil Broussard, Oct 22 2014
STATUS
approved