When I study the book "Opera De Cribro" by John Friedlander, Henryk Iwaniec-(2010), in Sections 1.2 and 1.3, I confused with notation used there. In page 3 it is defined:
$$\cal{A} = (a_n) , n\le x$$,
$$A(x)=\sum_{n\le x} a_n$$
$$\cal{A}_d=(am) , m \equiv 0 (\mod d)$$
and $$A_d(x) =\sum_{\substack{n\le x\\ n\equiv 0 (\mod d)}} a_n$$
Now my question happens in the example 1.1 on page 5.
what are $a_n$ here? I wonder if $a_n=1$ or $a_n=n$?
Thanks.
