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 A209870 Numbers n for which tau(n) = tau(n'), where tau is the number of divisors of n and n' the arithmetic derivative of n. 3
 4, 15, 21, 26, 27, 28, 33, 38, 48, 50, 57, 62, 69, 72, 74, 80, 85, 93, 99, 106, 129, 133, 134, 145, 156, 166, 176, 177, 178, 200, 205, 207, 213, 217, 218, 226, 237, 249, 253, 254, 262, 265, 276, 278, 308, 309, 314, 348, 362, 364, 368, 380, 393, 398, 410, 417 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Paolo P. Lava, Table of n, a(n) for n = 1..1000 EXAMPLE Divisors of 26 are four: 1, 2, 13 and 26. Arithmetic derivative of 26 is 15 and its divisors are four: 1, 3, 5 and 15. MAPLE with(numtheory); A209870:=proc(n) local a, i, p, pfs; for i from 1 to n do   pfs:=ifactors(i)[2]; a:=i*add(op(2, p)/op(1, p), p=pfs);   if tau(a)=tau(i) then print(i); fi; od; end: A209870(1000); CROSSREFS Cf. A000005, A003415, A190402, A190403 Sequence in context: A170850 A297711 A055465 * A167293 A267769 A192201 Adjacent sequences:  A209867 A209868 A209869 * A209871 A209872 A209873 KEYWORD nonn,easy AUTHOR Paolo P. Lava, Mar 15 2012 STATUS approved

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Last modified November 27 01:16 EST 2021. Contains 349344 sequences. (Running on oeis4.)