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A194082 Sum{floor(sqrt(3)*k/2) : 1<=k<=n} 1
0, 1, 3, 6, 10, 15, 21, 27, 34, 42, 51, 61, 72, 84, 96, 109, 123, 138, 154, 171, 189, 208, 227, 247, 268, 290, 313, 337, 362, 387, 413, 440, 468, 497, 527, 558, 590, 622, 655, 689, 724, 760, 797, 835, 873, 912, 952, 993, 1035, 1078, 1122, 1167, 1212 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Partial sums of A171970.

Comment from R. J. Mathar, Dec 02 2012 (Start):

a(n-1) is the number of unit squares regularly packed into the isosceles triangle of edge length n.

The triangle may be aligned with the Cartesian axes by putting its bottom edge on the horizontal axis, so its vertices are at (x,y) = (0,0), (n,0) and (n/2,sqrt(3)*n/2), see A010527.

The area inside the triangle is sqrt(3)*n^2/4 = A120011*n^2. There is an obvious upper limit of floor(sqrt(3)*n^2/4) = A171971(n) to the count of non-overlapping unit squares inside this triangle.

Regular packing: We place the first row of unit squares so they touch the bottom edge of the triangle. Their number is limited by the length of the horizontal section of the line y=1 inside the triangle, n-2*y/sqrt(3), which touches all of these first-row squares at their top.

The number of unit squares in the next row, between y=1 and y=2, is limited by the length of the horizontal section of the line y=2 inside the triangle, n-2*y/sqrt(3). Continuing, in row y=1, 2, ... we insert floor(n-2*y/sqrt(3)) unit squares, all with the same orientation.

The total number of squares is sum_{ y=1, 2, ..., floor(n*sqrt(3)/2) } floor( n-2*y/sqrt(3) ), and resummation yields, up to an index shift, this sequence here.

(End)

LINKS

Table of n, a(n) for n=1..53.

MATHEMATICA

r = Sqrt[3]/2;

c[k_] := Sum[Floor[j*r], {j, 1, k}];

Table[c[k], {k, 1, 90}]

PROG

(PARI) a(n)=sum(k=1, n, sqrtint(3*k^2\4)) \\ Charles R Greathouse IV, Jan 06 2013

CROSSREFS

Cf. A171970.

Sequence in context: A310081 A240443 A033439 * A061786 A171971 A184009

Adjacent sequences:  A194079 A194080 A194081 * A194083 A194084 A194085

KEYWORD

nonn

AUTHOR

Clark Kimberling, Aug 17 2011

STATUS

approved

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Last modified December 10 23:18 EST 2019. Contains 329910 sequences. (Running on oeis4.)