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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
OFFSET
1,1
LINKS
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
KEYWORD
nonn,easy
AUTHOR
Paolo P. Lava, Mar 15 2012
STATUS
approved