A061726 If n-th triangular number (A000217(n)) is odd, multiply it by 4; if even, multiply it by 5.
4, 12, 30, 50, 60, 84, 140, 180, 180, 220, 330, 390, 364, 420, 600, 680, 612, 684, 950, 1050, 924, 1012, 1380, 1500, 1300, 1404, 1890, 2030, 1740, 1860, 2480, 2640, 2244, 2380, 3150, 3330, 2812, 2964, 3900, 4100, 3444, 3612, 4730, 4950, 4140, 4324, 5640
Offset: 1
References
- Norman Sullivan, Test Your Own IQ, Workman Publishing Co. New York, NY, 1994, pp. 48, 50.
Links
- Harry J. Smith, Table of n, a(n) for n = 1..1000
Programs
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PARI
{ for (n=1, 1000, a=n*(n + 1)/2; if (a%2, a*=4, a*=5); write("b061726.txt", n, " ", a) ) } \\ Harry J. Smith, Jul 27 2009
Formula
a(n) = t*(5 - (t mod 2)) where t is the n-th triangular number (A000217(n)). - Jon E. Schoenfield, Jul 01 2018
G.f.: -2*(2*x^6+9*x^4-4*x^3+9*x^2+2)*x/((x-1)^3*(x^2+1)^3). - Alois P. Heinz, Jul 01 2018
Extensions
More terms from Larry Reeves (larryr(AT)acm.org), May 07 2001