A390911 - OEIS

%I #4 Nov 27 2025 21:43:26

%S 3,5,9,13,21,23,39,45,49,59,63,93,113,123,129,137,159,175,213,229,261,

%T 267,275,321,345,351,457,493,505,509,523,531,553,661,669,693,795,815,

%U 849,895,933,945,993,1005,1015,1021,1095,1263,1335,1347,1365,1633,1641,1643

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

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

%p with(NumberTheory):

%p A390911 := proc(N)

%p     local a,k,V,v;

%p     V := Vector(N);

%p     for k to max(20, ceil(N*log(N)^2)) do

%p         v := tau(k) + phi(k);

%p         if v <= N then

%p             V[v] := V[v] + 1

%p         fi

%p     od;

%p     V := convert(V, list);

%p     k := select(i -> V[i] > 0, [seq(1..nops(V), 2)]):

%p     op(k);

%p end proc:

%p A390911(1643);

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

%K nonn

%O 1,1

%A _Felix Huber_, Nov 23 2025