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 A218392 Primitive triangle numbers as defined in A218243. 2
 60, 150, 200, 780, 1530, 1690, 1950, 2040, 2100, 2730, 2860, 3570, 4056, 4200, 4350, 4624, 5100, 5460, 8120, 8250, 8670, 8750, 10812, 11050, 11900, 12180, 13260, 13650, 14060, 14430, 14760, 15540, 17220, 17850, 18564, 19240, 23400, 24420, 26100, 26520, 27880, 29172, 30000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Triangle numbers A218243 are integers n = a*b*c such that the triangle with sides (a,b,c) has integer area. For any such n, n*k^3 is again an element of A218243 for any integer k>0. Terms in A218243 which are not of that form (with k>1) are called "primitive", and listed here. LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..1000 EXAMPLE The term A218243(4)=480 is not in this sequence since 480 = 2^3*60 and 60 = A218243(1). The number n=30000=A218243(67) is divisible by 10^3, but neither of n/2^3, n/5^3, n/10^3 is in A218243, therefore n is in this sequence. PROG (PARI) is_A218392(n)=is_A218243(n) || return; my(f=factor(n)); vecmax(f[, 2])<3 || !fordiv(factorback(Mat(apply(t->[t[1], t[2]\3]~, Vec(f~)))~), d, d>1 & is_A218243(n/d^3) & return) CROSSREFS Sequence in context: A211332 A039497 A218243 * A135199 A112065 A252018 Adjacent sequences:  A218389 A218390 A218391 * A218393 A218394 A218395 KEYWORD nonn AUTHOR M. F. Hasler, Oct 27 2012 STATUS approved

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Last modified March 22 23:04 EDT 2019. Contains 321422 sequences. (Running on oeis4.)