A073725 - OEIS

A073725

a(n)-th composite number = phi(n-th composite number); a(1)=a(2)=0.

0, 0, 1, 2, 1, 1, 2, 3, 3, 2, 3, 6, 5, 3, 11, 6, 10, 6, 3, 9, 11, 9, 14, 6, 10, 14, 9, 6, 11, 14, 13, 9, 28, 11, 20, 14, 10, 27, 14, 24, 18, 9, 19, 24, 20, 32, 11, 20, 29, 14, 14, 24, 27, 24, 42, 14, 20, 37, 27, 14, 45, 28, 39, 27, 14, 51, 29, 42, 31, 51, 20, 28, 42, 27, 20, 32, 32

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,4

LINKS

Table of n, a(n) for n=1..77.

FORMULA

See program.

EXAMPLE

Phi of 25th composite number = 10th composite number: n=25: A002808(25)=38, phi(38) = 18 = A002808(10) so a(25)=10.

MATHEMATICA

c[x_] := FixedPoint[x+PrimePi[ # ]+1&, x]; G[x_] := x-PrimePi[x]-1; a(n)=G[EulerPhi[c[n]]]

CROSSREFS