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A069499 Triangular numbers of the form 21*k. 1
0, 21, 105, 210, 231, 378, 630, 861, 903, 1176, 1596, 1953, 2016, 2415, 3003, 3486, 3570, 4095, 4851, 5460, 5565, 6216, 7140, 7875, 8001, 8778, 9870, 10731, 10878, 11781, 13041, 14028, 14196, 15225, 16653, 17766, 17955, 19110, 20706, 21945, 22155, 23436, 25200 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Intersection of A000217 and A008603. - Michel Marcus, Sep 17 2013

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Index entries for linear recurrences with constant coefficients, signature (1,0,0,2,-2,0,0,-1,1).

FORMULA

G.f.: -21*x^2*(x^2-x+1)*(x^4+5*x^3+9*x^2+5*x+1) / ((x-1)^3*(x+1)^2*(x^2+1)^2). - Colin Barker, Sep 23 2013

MAPLE

a[0] := 0:a[1] := 6:a[2] := 14:a[3] := 20:a[4] := 21:a[5] := 27:a[6] := 35:a[7] := 41:seq((42*(floor(i/8))+a[i mod 8])*(42*(floor(i/8))+a[i mod 8]+1)/2, i=0..100);

MATHEMATICA

Select[21Range[1100], OddQ[Sqrt[8#+1]]&] (* Harvey P. Dale, Aug 16 2021 *)

CROSSREFS

Cf. A000217, A008603, A069498, A117748.

Sequence in context: A160082 A201468 A306259 * A068142 A126229 A060537

Adjacent sequences:  A069496 A069497 A069498 * A069500 A069501 A069502

KEYWORD

nonn,easy

AUTHOR

Amarnath Murthy, Mar 30 2002

EXTENSIONS

More terms from Sascha Kurz, Apr 01 2002

a(1)=0 added and edited by Alois P. Heinz, Aug 19 2021

STATUS

approved

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Last modified October 22 07:06 EDT 2021. Contains 348160 sequences. (Running on oeis4.)