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A068208
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Numbers n such that tau(n) - tau(n+1) = 1.
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4
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4, 8, 81, 441, 625, 1089, 2024, 2401, 3025, 3968, 5624, 5929, 6561, 6723, 7569, 8281, 8463, 12321, 13225, 13688, 14161, 14641, 14884, 15375, 16641, 20164, 21608, 24025, 25921, 26895, 28561, 34225, 46225, 55225, 55695, 61009, 62001, 67081
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OFFSET
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1,1
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COMMENTS
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a(n) cannot be a prime.
a(n) or a(n) + 1 is a perfect square as its number of divisors must be odd. - David A. Corneth, Dec 26 2020
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LINKS
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FORMULA
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MATHEMATICA
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Select[Range[10^5], DivisorSigma[0, #] - DivisorSigma[0, # + 1] == 1 &] (* Michael De Vlieger, Dec 02 2015 *)
Position[Differences[DivisorSigma[0, Range[70000]]], -1]//Flatten (* Harvey P. Dale, May 22 2020 *)
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PROG
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(PARI) isok(n) = numdiv(n) - numdiv(n+1) == 1; \\ Michel Marcus, Dec 02 2015
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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