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A137742 a(n) = (n-1)*(n+4)*(n+6)/6 for n>1, a(1)=1. 8
1, 8, 21, 40, 66, 100, 143, 196, 260, 336, 425, 528, 646, 780, 931, 1100, 1288, 1496, 1725, 1976, 2250, 2548, 2871, 3220, 3596, 4000, 4433, 4896, 5390, 5916, 6475, 7068, 7696, 8360, 9061, 9800, 10578, 11396, 12255, 13156, 14100, 15088, 16121, 17200, 18326, 19500 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also the number of different strings of length n+3 obtained from "123...n" by iteratively duplicating any substring (see A137743 for comments and examples). This is the principal (although not simplest) definition of this sequence and explains why a(1)=1 and not 0.

For n >= 3, sequence appears (not yet proved by induction) to give the number of multiplications between two nonzero matrix elements in calculating the product of two n X n Hessenberg matrices (square matrices which have 0s below the sub-diagonal, other elements being in general nonzero). - John M. Coffey, Jun 21 2016

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..10000

Index entries for doubling substrings

Index entries for linear recurrences with constant coefficients, signature (4,-6,4,-1).

FORMULA

From Bruno Berselli, Aug 23 2011:  (Start)

G.f.: x*(1+4*x-5*x^2+x^4)/(1-x)^4.

a(n) = +4*a(n-1) -6*a(n-2) +4*a(n-3) -a(n-4).

a(-n-7) = -A000297(n).  (End)

From Ilya Gutkovskiy, Jul 01 2016: (Start)

E.g.f.: 4 + x + (-24 + 24*x + 12*x^2 + x^3)*exp(x)/6.

Sum_{n>=1} 1/a(n) = 1542/1225. (End)

EXAMPLE

For n=5, a(n) = (5-1)*(5+4)*(5+6)/6 = 4*9*11/6 = 66. - Michael B. Porter, Jul 02 2016

MATHEMATICA

Join[{1}, Table[Binomial[n, 3]-2*n, {n, 6, 60}]] (*or*) Join[{1}, Table[(n-1)(n+4)(n+6)/6, {n, 2, 56}]] (* Vladimir Joseph Stephan Orlovsky, Apr 22 2011 *)

PROG

(PARI) A137742(n)=if(n<2, 1, n=A135473(n+3, n); n[ #n]) /* function A135473 defined in A137743 */

(PARI) A137742(n)=if(n<2, 1, (n - 1)*(n + 4)*(n + 6)/6)

(MAGMA) [1] cat [(n^3+9*n^2+14*n-24)/6: n in [2..46]];  // Bruno Berselli, Aug 23 2011

CROSSREFS

Cf. A137740-A137743, A135473, A137744-A137748.

See A275874 for another version.

Sequence in context: A225287 A000567 A124484 * A275874 A190456 A188026

Adjacent sequences:  A137739 A137740 A137741 * A137743 A137744 A137745

KEYWORD

nonn,easy

AUTHOR

M. F. Hasler, Feb 10 2008

STATUS

approved

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Last modified December 19 02:36 EST 2018. Contains 318245 sequences. (Running on oeis4.)