cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A218392 Primitive triangle numbers as defined in A218243.

Original entry on oeis.org

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
Offset: 1

Views

Author

M. F. Hasler, Oct 27 2012

Keywords

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.

Examples

			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.
		

Programs

  • 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)