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A065655
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Composite numbers n such that sigma(n)*phi(n) + 2*sigma(n) is a square.
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5
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28, 90, 156, 184, 374, 1855, 2162, 2170, 2280, 2376, 2415, 2665, 3160, 4970, 5270, 5740, 6402, 6494, 7414, 8400, 9118, 10656, 11155, 12400, 14632, 14910, 15010, 15906, 18183, 18792, 22648, 24645, 24734, 24920, 25844, 26670, 27478, 28990
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OFFSET
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1,1
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LINKS
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EXAMPLE
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Since (p+1)(p-1) + 2(p+1) = p^2 + 2p + 1 = (p+1)^2 is a square, all primes are solutions. For n = 28, sigma(28) = 56, phi(28) = 12, 56*12 + 2*56 = 784 = 28*28, so 28 is a composite solution.
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MATHEMATICA
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Select[Range@ 30000, Function[n, And[CompositeQ@ n, IntegerQ@ Sqrt[# EulerPhi@ n + 2 #] &@ DivisorSigma[1, n]]]] (* Michael De Vlieger, Mar 18 2017 *)
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PROG
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(PARI) { n=0; for (m=1, 10^9, if (isprime(m), next); s=sigma(m)*eulerphi(m) + 2*sigma(m); if (issquare(s), write("b065655.txt", n++, " ", m); if (n==500, return)) ) } \\ Harry J. Smith, Oct 25 2009
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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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