OFFSET
1,1
COMMENTS
p^2 and 2p^2 are always in inverse set, so a(n)>=2.
LINKS
Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems: invphi.gp, 2005-2019.
FORMULA
a(n) = Card[{InvPhi(p(n)*(p(n)-1)]} = Card[InvPhi(A036689(n)].
EXAMPLE
n=5: p(5)=11, a(5)=2 because InvPhi(110) = {121, 242}.
n=6: p(6)=13, a(6)=7 because InvPhi(13.12) = InvPhi(156)= {157, 169, 237, 314, 316, 338, 474}.
MAPLE
[seq(nops(invphi(ithprime(j)*(-1+ithprime(j)))), j=1..128)];
PROG
(PARI) a(n) = my(p=prime(n)); #invphi(p*(p-1)); \\ Michel Marcus, Mar 25 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Jun 04 2002
STATUS
approved