OFFSET
1,1
COMMENTS
Analog of the nontotients A005277.
Contains 2*p if p is in A307390. - Robert Israel, Apr 17 2019
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
2 is a term because there exists no m such that psi(m) = 2.
4 is not a term because 4 = 3*(3+1)/3.
MAPLE
N:= 1000: # to get all terms <= N
psi:= proc(n) local p; n*mul(1+1/p, p=numtheory:-factorset(n)) end proc:
sort(convert({seq(i, i=2..N, 2)} minus map(psi, {$1..N}), list)); # Robert Israel, Apr 17 2019
MATHEMATICA
M = 1000; (* to get all terms <= M *)
psi[n_] := n Sum[MoebiusMu[d]^2/d, {d, Divisors[n]}];
Range[2, M, 2] ~Complement~ (psi /@ Range[M]) (* Jean-François Alcover, Aug 01 2020, after Maple *)
PROG
(PARI) dpsi(n) = = n * sumdivmult(n, d, issquarefree(d)/d); \\ A001615
isok(n) = {if (!(n%2), for (k=1, n-1, if (dpsi(k) == n, return(0)); ); return (1); ); } \\ Michel Marcus, Mar 22 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Torlach Rush, Mar 21 2019
STATUS
approved