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

1,1

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

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

Prog

(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

Crossrefs

Cf. A000217, A061727.

Sequence in context: A309297 A296645 A280007 * A067706 A008261 A099407

Adjacent sequences: A061723 A061724 A061725 * A061727 A061728 A061729

Keyword

easy,nonn

Author

Jason Earls, May 05 2001

Extensions

More terms from Larry Reeves (larryr(AT)acm.org), May 07 2001

Status

approved