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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 February 20 08:00 EST 2020. Contains 332069 sequences. (Running on oeis4.)