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 A168380 Row sums of A168281. 14
 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. 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 * A308286 A297184 A218871 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 July 21 04:40 EDT 2019. Contains 325189 sequences. (Running on oeis4.)