OFFSET
1,1
COMMENTS
a(1) = 710533 = 487 * 1459 has 2 distinct prime divisors.
a(2) = 915981 = 3 * 11 * 41 * 677 has 4 distinct prime divisors.
a(58) = 176529862601 = 2141 * 6421 * 12841 is the least term with 3 distinct prime divisors.
a(6884) = 15314196673937701 = 19 * 31 * 41 * 71 * 109 * 281 * 331 * 881 is the least term with 8 distinct prime divisors.
a(111) = 619303584901 is the least term k such that A003961(k) is also a term.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
710533 = 487 * 1459 is a term since it is a Fermat pseudoprime to base 2, and A003961(710533) = 491 * 1471 = 722261 is also a Fermat pseudoprime to base 2.
MATHEMATICA
psp = Cases[Import["https://oeis.org/A001567/b001567.txt", "Table"], {_, _}][[;; , 2]]; f[p_, e_] := NextPrime[p]^e; s[1] = 1; s[n_] := Times @@ f @@@ FactorInteger[n]; pspQ[n_] := PowerMod[2, n - 1, n] == 1; Select[psp, pspQ[s[#]] &]
CROSSREFS
KEYWORD
nonn
AUTHOR
Amiram Eldar, Jul 23 2021
STATUS
approved