Can you tell what this does without running it?

from itertools import *
from operator import *

def fs():
  return filter(f, chain(range(2), islice(count(), 1, None)))

def f(n):
  return [1 for o in [4, -4] if p(5*n*n + o)]

def p(n):
  return [d for d in range(n) if d*d == n]

for n in fs():
  print(n)

(Run with Python 3.)