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 A099040 Riordan array (1,2+2x). 2
 1, 0, 2, 0, 2, 4, 0, 0, 8, 8, 0, 0, 4, 24, 16, 0, 0, 0, 24, 64, 32, 0, 0, 0, 8, 96, 160, 64, 0, 0, 0, 0, 64, 320, 384, 128, 0, 0, 0, 0, 16, 320, 960, 896, 256, 0, 0, 0, 0, 0, 160, 1280, 2688, 2048, 512, 0, 0, 0, 0, 0, 32, 960, 4480, 7168, 4608, 1024, 0, 0, 0, 0, 0, 0, 384, 4480, 14336, 18432, 10240, 2048 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Row sums give A002605. Diagonal sums give A052907. The Riordan array (1,s+t*x) defines T(n,k)=binomial(k,n-k)s^k(t/s)^(n-k). The row sums satisfy a(n) = s*a(n-1) + t*a(n-2) and the diagonal sums satisfy a(n) = s*a(n-2) + t*a(n-3). T(n,k) is the number of compositions of n into two types of parts of size 1 and 2 that have exactly k parts. - Geoffrey Critzer, Aug 18 2012. LINKS G. C. Greubel, Table of n, a(n) for the first 100 rows, flattened FORMULA Number triangle T(n, k) = 2^k*binomial(k, n-k). Columns have g.f. (2x+2x^2)^k. T(n,k) = A026729(n,k)*2^k . - Philippe Deléham, Jul 28 2006 O.g.f.: 1/(1-2*y*x-2*y*x^2) - Geoffrey Critzer, Aug 18 2012. EXAMPLE Rows begin {1}, {0,2}, {0,2,4}, {0,0,8,8}, {0,0,4,24,16}, {0,0,0,24,64,32},... T(3,2)=8 because we have: 1+2,1+2',1'+2,1'+2',2+1,2+1',2'+1,2'+1' where a part of the second type is designated by '. - Geoffrey Critzer, Aug 18 2012 MATHEMATICA nn = 8; CoefficientList[Series[1/(1 - 2 y x - 2 y x^2), {x, 0, nn}], {x, y}] // Grid  (* Geoffrey Critzer, Aug 18 2012 *) CROSSREFS Cf. A002605, A026729, A038208, A052907. Sequence in context: A206823 A151668 A086151 * A185296 A136717 A261685 Adjacent sequences:  A099037 A099038 A099039 * A099041 A099042 A099043 KEYWORD easy,nonn,tabl AUTHOR Paul Barry, Sep 23 2004 STATUS approved

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Last modified October 16 13:32 EDT 2019. Contains 328093 sequences. (Running on oeis4.)