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EXAMPLE
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For n=1, sigma(1) = 1, both gcd(1-0, 1-(1-0)) = gcd(1,0) = 1 and gcd(1-1, 1-(1-1)) = gcd(0,1) = 1, thus a(1) = 2.
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For n=3, sigma(3) = 4, we have 5 cases to consider:
gcd(3-0, 3-(4-0)) = 1 = gcd(3-4, 3-(4-4)),
gcd(3-1, 3-(4-1)) = 2 = gcd(3-3, 3-(4-3)),
gcd(3-2, 3-(4-2)) = 1,
of which three cases give 1 as a result, thus a(3) = 3.
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For n=6, sigma(6) = 12, we have 13 cases to consider:
gcd(6-0, 6-(12-0)) = 6 = gcd(6-12, 6-(12-12)),
gcd(6-1, 6-(12-1)) = 5 = gcd(6-11, 6-(12-11)),
gcd(6-2, 6-(12-2)) = 4 = gcd(6-10, 6-(12-10)),
gcd(6-3, 6-(12-3)) = 3 = gcd(6-9, 6-(12-9)),
gcd(6-4, 6-(12-4)) = 2 = gcd(6-8, 6-(12-8))
gcd(6-5, 6-(12-5)) = 1 = gcd(6-7, 6-(12-7)),
gcd(6-6, 6-(12-6)) = 0,
of which only two give 1 as a result, thus a(6) = 2.
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For n=10, sigma(10) = 18, we have 19 cases to consider:
gcd(10-0, 10-(18-0)) = 2 = gcd(10-18, 10-(18-18)),
gcd(10-1, 10-(18-1)) = 1 = gcd(10-17, 10-(18-17)),
gcd(10-2, 10-(18-2)) = 2 = gcd(10-16, 10-(18-16)),
gcd(10-3, 10-(18-3)) = 1 = gcd(10-15, 10-(18-15)),
gcd(10-4, 10-(18-4)) = 2 = gcd(10-14, 10-(18-14)),
gcd(10-5, 10-(18-5)) = 1 = gcd(10-13, 10-(18-13)),
gcd(10-6, 10-(18-6)) = 2 = gcd(10-12, 10-(18-12)),
gcd(10-7, 10-(18-7)) = 1 = gcd(10-11, 10-(18-11)),
gcd(10-8, 10-(18-8)) = 2 = gcd(10-10, 10-(18-10)),
gcd(10-9, 10-(18-9)) = 1,
of which 9 cases give 1 as a result, thus a(10) = 9.
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For n=15, sigma(15) = 24, we have 25 cases to consider:
gcd(15-0, 15-(24-0)) = 3 = gcd(15-24, 15-(24-24)),
gcd(15-1, 15-(24-1)) = 2 = gcd(15-23, 15-(24-23)),
gcd(15-2, 15-(24-2)) = 1 = gcd(15-22, 15-(24-22)),
gcd(15-3, 15-(24-3)) = 6 = gcd(15-21, 15-(24-21)),
gcd(15-4, 15-(24-4)) = 1 = gcd(15-20, 15-(24-20)),
gcd(15-5, 15-(24-5)) = 2 = gcd(15-19, 15-(24-19)),
gcd(15-6, 15-(24-6)) = 3 = gcd(15-18, 15-(24-18)),
gcd(15-7, 15-(24-7)) = 2 = gcd(15-17, 15-(24-17)),
gcd(15-8, 15-(24-8)) = 1 = gcd(15-16, 15-(24-16)),
gcd(15-9, 15-(24-9)) = 6 = gcd(15-15, 15-(24-15)),
gcd(15-10, 15-(24-10)) = 1 = gcd(15-14, 15-(24-14)),
gcd(15-11, 15-(24-11)) = 2 = gcd(15-13, 15-(24-13)),
gcd(15-12, 15-(24-12)) = 3,
of which 2*4 = 8 cases give 1 as a result, thus a(15) = 8.
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