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A171452 a(n) = C(n,2) + floor(n/3). 3
0, 0, 1, 4, 7, 11, 17, 23, 30, 39, 48, 58, 70, 82, 95, 110, 125, 141, 159, 177, 196, 217, 238, 260, 284, 308, 333, 360, 387, 415, 445, 475, 506, 539, 572, 606, 642, 678, 715, 754, 793, 833, 875, 917, 960, 1005, 1050, 1096, 1144, 1192, 1241, 1292, 1343, 1395 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

Exponents in Hankel transform A171451.

For n>=2 a(n) is the smallest addend in the sums of n terms where all the natural numbers are used once 1+2=3, 4+5+6=15, 7+8+9+10=34, 11+12+13+14+16=66, 17+18+19+20+21+22=117 23+24+25+26+27+28+29=182, 30+31+32+33+35+36+37+38=272. - Anton Zakharov, Aug 28 2016

LINKS

Table of n, a(n) for n=0..53.

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

FORMULA

G.f.: (1+2x)/((1-x)^3*(1+x+x^2));

a(n)=(3n^2-n-2)/6+sqrt(3)*cos(2*pi*n/3+pi/6)/9+sin(2*pi*n/3+pi/6)/3.

PROG

(PARI) a(n)=n*(3*n-1)\6 \\ Charles R Greathouse IV, Aug 28 2016

CROSSREFS

Cf. A130481 (first differences)

Sequence in context: A310766 A097403 A126618 * A049648 A211647 A133257

Adjacent sequences:  A171449 A171450 A171451 * A171453 A171454 A171455

KEYWORD

easy,nonn

AUTHOR

Paul Barry, Dec 09 2009

STATUS

approved

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Last modified October 18 21:56 EDT 2021. Contains 348070 sequences. (Running on oeis4.)