OFFSET
1,1
COMMENTS
Also numbers k such that the noncentral divisors of k are 1 and k.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
4 is in the sequence because 4 has three divisors, they are 1, 2, 4. On the other hand, the noncentral divisors of 4 are 1 and 4, in accordance with the first comment.
6 is in the sequence because 6 has four divisors, they are 1, 2, 3, 6. On the other hand, the noncentral divisors of 6 are 1 and 6, in accordance with the first comment.
MATHEMATICA
Select[Range[200], MemberQ[{3, 4}, DivisorSigma[0, #]] &] (* Amiram Eldar, Dec 03 2020 *)
PROG
(PARI) isok(n) = my(nd=numdiv(n)); (nd==3) || (nd==4); \\ Michel Marcus, Feb 26 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Omar E. Pol, Feb 26 2019
STATUS
approved