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A379094
Numbers whose factors in the canonical prime factorization neither increase weakly nor decrease weakly.
4
60, 84, 90, 120, 126, 132, 156, 168, 180, 204, 228, 240, 252, 264, 270, 276, 280, 300, 312, 315, 336, 348, 350, 360, 372, 378, 408, 420, 440, 444, 456, 480, 492, 495, 504, 516, 520, 525, 528, 540, 550, 552, 560, 564, 585, 588, 594, 600, 616, 624, 630, 636, 650
OFFSET
1,1
COMMENTS
A379097 is a subsequence.
From Michael De Vlieger, Dec 18 2024: (Start)
Proper subset of A126706.
Smallest powerful number is a(314) = 2700. (End)
LINKS
EXAMPLE
60 is a term because the factors in the canonical prime factorization are [4, 3, 5], a list that is neither increasing nor decreasing.
Primorials (A002110) are not terms of this sequence.
MAPLE
with(ArrayTools):
fact := n -> local p; [seq(p[1]^p[2], p in ifactors(n)[2])]:
isA379094 := proc(n) local f; f := fact(n);
is(not IsMonotonic(f, direction=decreasing, strict=false) and not IsMonotonic(f, direction=increasing, strict=false)) end:
select(isA379094, [seq(1..650)]);
MATHEMATICA
Select[Range[650], Function[f, NoneTrue[{Sort[f], ReverseSort[f]}, # == f &]][Power @@@ FactorInteger[#]] &] (* Michael De Vlieger, Dec 18 2024 *)
PROG
(PARI) is_a379094(n) = my(C=apply(x->x[1]^x[2], Vec(factor(n)~))); vecsort(C)!=C && vecsort(C, , 4)!=C \\ Hugo Pfoertner, Dec 18 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
Peter Luschny, Dec 17 2024
STATUS
approved