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A336562
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Number of pairs sigma(p^x), sigma(q^y) that are not coprime, where p^x and q^y are any two maximal prime power divisors of n, with p < q. a(1) = 0.
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4
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0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 1, 1, 1, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0, 1, 0, 0, 0, 0, 1, 2, 0, 0, 1, 2, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 1, 0, 0, 1, 0, 0, 0, 2, 0, 0, 3
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OFFSET
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1,30
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LINKS
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EXAMPLE
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For n = 40 = 2^3 * 5^1, sigma(2^3) = 15 and sigma(5) = 6, so we have one such prime power pair that the gcd of their sum of divisors is larger than one (in this case gcd(15,6) = 3), thus a(40) = 1.
For n = 120 = 2^3 * 3^1 * 5^1, possible pairs are [sigma(8), sigma(3)], [sigma(8), sigma(5)] and [sigma(3), sigma(5)], with gcd(15,4) = 1, gcd(15,6) = 3 and gcd(4,6) = 2, thus there are two pairs that are not coprime, and a(120) = 2.
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PROG
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(PARI) A336562(n) = if(1==n, 0, my(f=factor(n), s=0); for(i=1, #f~, for(j=1+i, #f~, if(1!=gcd(sigma(f[i, 1]^f[i, 2]), sigma(f[j, 1]^f[j, 2])), s++))); (s));
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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