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(Python)
from itertools import count, islice
from sympy import factorint
def A046711_gen(): # generator of terms
return filter(lambda n:0 < n & 3 < 3 and all(p & 3 != 3 or e & 1 == 0 for p, e in factorint(n).items()), count(0))
A046711_list = list(islice(A046711_gen(), 30)) # Chai Wah Wu, Jun 28 2022
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If n == 1 or 2 (mod 4) and the squarefree part of n is divisible by a prime p == 3 (mod 4), then no difference set of order n exists. Equivalently, if a projective plane of order n exists, and n == 1 or 2 (mod 4), then n is the sum of two squares. - Jonathan Vos Post, Apr 17 2011
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reviewed
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proposed
If n = = 1, or 2 (mod 4) and the squarefree part of n is divisible by a prime p = = 3 (mod 4), then no difference set of order n exists. Equivalently, if a projective plane of order n exists, and n == 1 or 2 (mod 4), then n is the sum of two squares. - Jonathan Vos Post, Apr 17 2011
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editing