A070547 a(n) = Card(k, 0<k<n such that phi(k) divides phi(n-k)).

0, 1, 2, 2, 2, 3, 4, 4, 5, 6, 5, 5, 7, 6, 7, 8, 7, 9, 5, 10, 9, 10, 9, 7, 11, 9, 10, 10, 8, 9, 10, 11, 13, 12, 12, 13, 11, 11, 12, 14, 12, 13, 8, 16, 15, 16, 15, 11, 19, 16, 12, 15, 11, 16, 15, 17, 16, 15, 10, 14, 15, 12, 20, 15, 20, 18, 11, 17, 16, 18, 16, 18, 15, 18, 18, 16, 18

Offset

1,3

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Maple

f:= proc(n)
  nops(select(i -> numtheory:-phi(n-i) mod numtheory:-phi(i) = 0, [$1..n-1]))
end proc:
map(f, [$1..100]); # Robert Israel, Apr 01 2025

Prog

(PARI) a(n) = sum(i=1, n-1, if(eulerphi(n-i)%eulerphi(i), 0, 1));

Crossrefs

Cf. A000010.

Sequence in context: A303003 A063124 A358901 * A319922 A289139 A094838

Adjacent sequences: A070544 A070545 A070546 * A070548 A070549 A070550

Keyword

easy,nonn

Author

Benoit Cloitre, May 02 2002

Status

approved