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 A260577 Numbers n for which d(n+d(n)) < d(n), where d(n) is the number of divisors of n. 3
 4, 15, 16, 20, 21, 24, 27, 28, 30, 32, 33, 36, 39, 42, 45, 48, 52, 54, 55, 56, 57, 63, 64, 66, 68, 69, 75, 76, 78, 81, 85, 90, 93, 100, 105, 110, 112, 114, 116, 117, 120, 123, 126, 133, 135, 138, 140, 144, 145, 150, 153, 159, 160, 162, 165, 168, 170, 171, 172 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms are composite. Indeed, if p is prime then d(p)=2 will never be larger than d(p+d(p)) = d(p+2). - M. F. Hasler, Jul 30 2015 Conjecture: for every x>=6, among the first x terms, the terms divisible by 3 are never in the minority. Let A(y) be the number of terms <= y, y>=1. If the conjecture is true, then, taking into account the initials, we conclude that always A(y) < (2/3)*y. - Vladimir Shevelev, Jul 31 2015 LINKS Peter J. C. Moses, Table of n, a(n) for n = 1..2000 EXAMPLE 75 is in the sequence, since d(75) = 6 > d(75+6) = 5. PROG (PARI) is(n)=numdiv(n+n=numdiv(n))

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Last modified February 24 02:58 EST 2024. Contains 370288 sequences. (Running on oeis4.)