d(16) = 5. So a(16) is the number of integers coprime to 16 which are <= 5. There are 3 such integers: 1, 3, 5; so a(16) = 3.

MAPLE

with(numtheory): a:=proc(n) local ct, j: ct:=0: for j from 1 to tau(n) do if gcd(j, n)=1 then ct:=ct+1 else fi od: ct; end: seq(a(n), n=1..140); # Emeric Deutsch, Apr 02 2007