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A061777 Start with a single triangle; at n-th generation add a triangle at each vertex, allowing triangles to overlap; sequence gives total population of triangles at n-th generation. 16
1, 4, 10, 22, 40, 70, 112, 178, 268, 406, 592, 874, 1252, 1822, 2584, 3730, 5260, 7558, 10624, 15226, 21364, 30574, 42856, 61282, 85852, 122710, 171856, 245578, 343876, 491326, 687928, 982834, 1376044, 1965862, 2752288, 3931930, 5504788 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

From the definition, assign label value "1" to an origin triangle; at n-th generation add a triangle at each vertex. Each non-overlapping triangle will have the same label value as that of the predecessor triangle to which it is connected; for the overlapping ones, the label value will be the sum of the label values of predecessors. a(n) is the sum of all label values at the n-th generation. The triangle count is A005448. See illustration. For n >= 1, (a(n) - a(n-1))/3 is A027383. - Kival Ngaokrajang, Sep 05 2014

REFERENCES

R. Reed, The Lemming Simulation Problem, Mathematics in School, 3 (#6, Nov. 1974), front cover and pp. 5-6.

LINKS

Table of n, a(n) for n=0..36.

Kival Ngaokrajang, Illustration of initial terms

R. Reed, The Lemming Simulation Problem, Mathematics in School, 3 (#6, Nov. 1974), front cover and pp. 5-6. [Scanned photocopy of pages 5, 6 only, with annotations by R. K. Guy and N. J. A. Sloane]

Index entries for linear recurrences with constant coefficients, signature (2,1,-4,2).

FORMULA

If n even, 21*2^(n/2)-6*n-20; if n odd, 30*2^((n-1)/2)-6*(n-1)-26.

a(n) = 2*a(n-1)+a(n-2)-4*a(n-3)+2*a(n-4). G.f.: (1+2*x)*(1+x^2)/((1-x)^2*(1-2*x^2)). - Colin Barker, May 08 2012

a(n) = -20 - 6*n + (21+15*sqrt(2))*sqrt(2)^(n-2) + (21-15*sqrt(2))*(-sqrt(2))^(n-2). a(n) = 2*a(n-2) + ((3*n-2)/(3*n-5))*(a(n-1)-2*a(n-3)). - Robert Israel, Sep 14 2014

MAPLE

seq(`if`(n::even, 21*2^(n/2) - 6*n-20, 30*2^((n-1)/2)-6*n-20), n=0..100); # Robert Israel, Sep 14 2014

MATHEMATICA

Table[If[EvenQ[n], 21 2^(n/2)-6n-20, 30 2^((n-1)/2)-6(n-1)-26], {n, 0, 40}] (* Harvey P. Dale, Nov 06 2011 *)

PROG

(PARI) a(n)=if(n%2, 30, 21)<<(n\2) - 6*n - 20 \\ Charles R Greathouse IV, Sep 19 2014

CROSSREFS

Partial sums of A061776.

Cf. A005448, A027383. - Kival Ngaokrajang, Sep 19 2014

Sequence in context: A008248 A301243 A177736 * A298030 A155369 A155404

Adjacent sequences:  A061774 A061775 A061776 * A061778 A061779 A061780

KEYWORD

nonn,nice,easy

AUTHOR

N. J. A. Sloane, R. K. Guy, Jun 23 2001

EXTENSIONS

Corrected by T. D. Noe, Nov 08 2006

STATUS

approved

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Last modified June 25 12:01 EDT 2019. Contains 324352 sequences. (Running on oeis4.)