OFFSET
1,2
COMMENTS
Since Sum_{d|k} phi(d) = k, these are numbers k such that the set {phi(d) | d|k} is a partition of k into distinct parts.
Includes all the odd prime numbers, since an odd prime p has 2 divisors, 1 and p, whose phi values are 1 and p-1.
If k is a term, then all the divisors of k are also terms. If k is not a term, then all its multiples are not terms. The primitive terms of the complementary sequence are 2, 63, 273, 513, 585, 825, 2107, 2109, 2255, 3069, ....
In particular, all the terms are odd since 2 is not a term (phi(1) = phi(2)).
The number of terms below 10^k for k = 1, 2, ... are 5, 49, 488, 4860, 48598, 485807, 4857394, 48572251, 485716764, 4857144075, ...
Apparently the sequence has an asymptotic density of 0.4857...
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
3 is a term since it has 2 divisors, 1 and 3, and phi(1) = 1 != phi(3) = 2.
15 is a term since the phi values of its divisors, {1, 3, 5, 15}, are distinct: {1, 2, 4, 8}.
MAPLE
filter:= proc(n) local D;
D:=numtheory:-divisors(n);
nops(D) = nops(map(numtheory:-phi, D))
end proc:
select(filter, [seq(i, i=1..200, 2)]); # Robert Israel, Oct 29 2019
MATHEMATICA
aQ[n_] := Length @ Union[EulerPhi /@ (d = Divisors[n])] == Length[d]; Select[Range[130], aQ]
PROG
(PARI) isok(k) = #Set(apply(x->eulerphi(x), divisors(k))) == numdiv(k); \\ Michel Marcus, Oct 28 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Amiram Eldar, Oct 28 2019
STATUS
approved