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A168380 Row sums of A168281. 19
2, 4, 12, 20, 38, 56, 88, 120, 170, 220, 292, 364, 462, 560, 688, 816, 978, 1140, 1340, 1540, 1782, 2024, 2312, 2600, 2938, 3276, 3668, 4060, 4510, 4960, 5472, 5984, 6562, 7140, 7788, 8436, 9158, 9880, 10680, 11480, 12362, 13244, 14212, 15180, 16238, 17296, 18448, 19600, 20850, 22100 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The atomic numbers of the augmented alkaline earth group in Charles Janet's spiral periodic table are 0 and the first eight terms of this sequence (see Stewart reference). - Alonso del Arte, May 13 2011

Maximum number of 123 patterns in an alternating permutation of length n+3. - Lara Pudwell, Jun 09 2019

LINKS

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

Lara Pudwell, Packing patterns in restricted permutations, 2019.

Lara Pudwell, From permutation patterns to the periodic table, Notices of the American Mathematical Society. 67.7 (2020), 994-1001.

Philip Stewart, Charles Janet: unrecognized genius of the Periodic System. Foundations of Chemistry (2010), p. 9.

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

FORMULA

a(n) = 2*A005993(n-1).

a(n) = (n+1)*(3 + 2*n^2 + 4*n - 3*(-1)^n)/12.

a(n+1) - a(n) = A093907(n) = A137583(n+1).

a(2n+1) = A035597(n+1), a(2n) = A002492(n).

a(n) = A099956(n-1), 2 <= n <= 7.

a(n) = 2*a(n-1) + a(n-2) - 4*a(n-3) + a(n-4) + 2*a(n-5) - a(n-6).

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

a(n) = A000292(n) + A027656(n-1). - Paul Curtz, Oct 26 2012

E.g.f.: (1/12)*(3*(x - 1) + (3 + 15*x + 12*x^2 + 2*x^3)*exp(2*x))*exp(-x). - G. C. Greubel, Jul 19 2016

EXAMPLE

From Lara Pudwell, Jun 09 2019: (Start)

a(1)=2. The alternating permutation of length 1+3=4 with the maximum number of copies of 123 is 1324.  The two copies are 124 and 134.

a(2)=4.  The alternating permutation of length 2+3=5 with the maximum number of copies of 123 is 13254.  The four copies are 124, 125, 134, and 135.

a(3)=12. The alternating permutation of length 3+3=6 with the maximum number of copies of 123 is 132546.  The twelve copies are 124, 125, 126, 134, 135, 136, 146, 156, 246, 256, 346, and 356. (End)

MATHEMATICA

LinearRecurrence[{2, 1, -4, 1, 2, -1}, {2, 4, 12, 20, 38, 56}, 50] (* G. C. Greubel, Jul 19 2016 *)

Table[(n + 1) (3 + 2 n^2 + 4 n - 3 (-1)^n)/12, {n, 50}] (* Michael De Vlieger, Jul 20 2016 *)

PROG

(MAGMA) [(n+1)*(3+2*n^2+4*n-3*(-1)^n)/12: n in [1..50] ]; // Vincenzo Librandi, Aug 06 2011

(PARI) a(n)=([0, 1, 0, 0, 0, 0; 0, 0, 1, 0, 0, 0; 0, 0, 0, 1, 0, 0; 0, 0, 0, 0, 1, 0; 0, 0, 0, 0, 0, 1; -1, 2, 1, -4, 1, 2]^(n-1)*[2; 4; 12; 20; 38; 56])[1, 1] \\ Charles R Greathouse IV, Jul 21 2016

CROSSREFS

Sequence in context: A090922 A056228 A166869 * A335730 A308286 A297184

Adjacent sequences:  A168377 A168378 A168379 * A168381 A168382 A168383

KEYWORD

nonn,easy

AUTHOR

Paul Curtz, Nov 24 2009

STATUS

approved

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Last modified June 26 20:24 EDT 2022. Contains 354885 sequences. (Running on oeis4.)