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+ 209 = 11 x 19 is a member of a series n(n + 1) - 1 with the property that all terms greater than 1 are divisible by primes of form 10k + 1 or 10k - 1, but not 10k + 3 or 10k - 3. There is an infinity of pairs x,y where x2 - y2 - xy = 209 for x, y integer. [McNamara]




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