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 A064000 Unitary untouchable numbers of second kind: numbers n such that usigma(x) = n has no solution, where usigma(x) (A034448) is the sum of unitary divisors of x. 7
 2, 7, 11, 13, 15, 16, 19, 21, 22, 23, 25, 27, 29, 31, 34, 35, 37, 39, 41, 43, 45, 46, 47, 49, 51, 52, 53, 55, 57, 58, 59, 61, 63, 64, 66, 67, 69, 71, 73, 75, 76, 77, 79, 81, 83, 85, 86, 87, 88, 89, 91, 92, 93, 94, 95, 97, 99, 101, 103, 105, 106, 107, 109, 111, 113, 115, 116 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Donovan Johnson, Table of n, a(n) for n = 1..10000 C. Pomerance and H.-S. Yang, On untouchable numbers and related problems, 2012 C. Pomerance and H.-S. Yang, Variant of a theorem of Erdos on the sum-of-proper-divisors function, 2012 FORMULA Suppose usigma(x)=n. Then by definition usigma(x)=n>1 for n>1. Let x be a prime. Then usigma(x)=x+1 and so n=x+1. For x not prime, of course, x+1

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Last modified April 15 17:25 EDT 2021. Contains 342977 sequences. (Running on oeis4.)