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A357590
Triangular numbers which are products of five distinct primes (or pentaprimes).
0
3570, 8778, 9870, 12090, 13530, 20706, 20910, 21945, 24090, 24310, 26565, 33670, 40470, 40755, 47586, 54285, 57630, 57970, 63546, 66430, 69006, 72390, 76245, 87990, 88410, 91806, 92235, 94395, 94830, 98790, 121278, 130305, 132870, 133386, 141778, 148785, 154290, 159330, 163878, 167910
OFFSET
1,1
COMMENTS
A squarefree subsequence of triangular numbers (T(n) = n*(n+1)/2).
EXAMPLE
3570 = 2*3*5*7*17 = 84*85/2.
21945 = 3*5*7*11*19 = 209*210/2.
121278 = 2*3*17*29*41 = 492*493/2.
154290 = 2*3*5*37*139 = 555*556/2.
MAPLE
q:= n-> map(i-> i[2], ifactors(n)[2])=[1$5]:
select(q, [seq(n*(n+1)/2, n=0..1000)])[]; # Alois P. Heinz, Oct 05 2022
MATHEMATICA
Select[Accumulate @ Range[600], FactorInteger[#][[;; , 2]] == {1, 1, 1, 1, 1} &] (* Amiram Eldar, Oct 05 2022 *)
CROSSREFS
Intersection of A000217 and A046387.
Sequence in context: A204600 A204599 A206090 * A071144 A204417 A204410
KEYWORD
nonn
AUTHOR
Massimo Kofler, Oct 05 2022
STATUS
approved