Sum of a series

In general, the $nth partial sum of a series whose terms are given by a unary function &f is

[+] map &f, 1 .. $n

So what's needed in this case is

say [+] map { 1 / $^n**2 }, 1 .. 1000;

Or, using the "hyper" metaoperator to vectorize, we can use a more "point free" style while keeping traditional precedence:

say [+] 1 «/« (1..1000) »**» 2;

Or we can use the X "cross" metaoperator, which is convenient even if one side or the other is a scalar. In this case, we demonstrate a scalar on either side:

say [+] 1 X/ (1..1000 X** 2);

Note that cross ops are parsed as list infix precedence rather than using the precedence of the base op as hypers do. Hence the difference in parenthesization.

With list comprehensions, you can write:

say [+] (1 / $_**2 for 1..1000);

That's fine for a single result, but if you're going to be evaluating the sequence multiple times, you don't want to be recalculating the sum each time, so it's more efficient to define the sequence as a constant to let the run-time automatically cache those values already calculated. In a lazy language like Raku, it's generally considered a stronger abstraction to write the correct infinite sequence, and then take the part of it you're interested in. Here we define an infinite sequence of partial sums (by adding a backslash into the reduction to make it look "triangular"), then take the 1000th term of that:

constant @x = [\+] 0, { 1 / ++(state $n) ** 2 } ... *;
say @x[1000];  # prints 1.64393456668156

Note that infinite constant sequences can be lazily generated in Raku, or this wouldn't work so well...

A cleaner style is to combine these approaches with a more FP look:

constant ζish = [\+] map -> \𝑖 { 1 / 𝑖**2 }, 1..*;
say ζish[1000];

Perhaps the cleanest way is to just define the zeta function and evaluate it for s=2, possibly using memoization:

use experimental :cached;
sub ζ($s) is cached { [\+] 1..* X** -$s }
say ζ(2)[1000];

Notice how the thus-defined zeta function returns a lazy list of approximated values, which is arguably the closest we can get from the mathematical definition.