A390911 Odd numbers k for which tau(x) + phi(x) = k has at least one solution.

3, 5, 9, 13, 21, 23, 39, 45, 49, 59, 63, 93, 113, 123, 129, 137, 159, 175, 213, 229, 261, 267, 275, 321, 345, 351, 457, 493, 505, 509, 523, 531, 553, 661, 669, 693, 795, 815, 849, 895, 933, 945, 993, 1005, 1015, 1021, 1095, 1263, 1335, 1347, 1365, 1633, 1641, 1643

Offset

1,1

Comments

Odd numbers k for which A390872(k) > 0.

Links

Table of n, a(n) for n=1..54.

Maple

with(NumberTheory):
A390911 := proc(N)
    local a, k, V, v;
    V := Vector(N);
    for k to max(20, ceil(N*log(N)^2)) do
        v := tau(k) + phi(k);
        if v <= N then
            V[v] := V[v] + 1
        fi
    od;
    V := convert(V, list);
    k := select(i -> V[i] > 0, [seq(1..nops(V), 2)]):
    op(k);
end proc:
A390911(1643);

Crossrefs

Cf. A000005, A000010, A014197, A100966, A390872.

Sequence in context: A007042 A178415 A249424 * A076274 A058989 A049691

Adjacent sequences: A390908 A390909 A390910 * A390912 A390913 A390914

Keyword

nonn

Author

Felix Huber, Nov 23 2025

Status

approved