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 A005993 G.f.: (1+x^2)/((1-x)^2*(1-x^2)^2). (Formerly M1576) 35

%I M1576

%S 1,2,6,10,19,28,44,60,85,110,146,182,231,280,344,408,489,570,670,770,

%T 891,1012,1156,1300,1469,1638,1834,2030,2255,2480,2736,2992,3281,3570,

%U 3894,4218,4579,4940,5340,5740,6181,6622,7106,7590,8119,8648,9224,9800

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

%C Alkane (or paraffin) numbers l(6,n).

%C Dimension of the space of homogeneous degree n polynomials in (x1, y1, x2, y2) invariant under permutation of variables x1<->y1, x2<->y2.

%C Also multidigraphs with loops on 2 nodes with n arcs (see A138107). - _Vladeta Jovovic_, Dec 27 1999

%C Euler transform of finite sequence [2,3,0,-1]. - _Michael Somos_, Mar 17 2004

%C a(n-2) is the number of plane partitions with trace 2. - _Michael Somos_, Mar 17 2004

%C With offset 4, a(n) is the number of bracelets with n beads, 3 of which are red, 1 of which is blue. For odd n, a(n) = C(n-1,3)/2. For even n, a(n) = C(n-1,3)/2 +(n-2)/4. For n >= 6, with K = (n-1)(n-2)/((n-5)(n-4)), for odd n, a(n) = K*a(n-2). For even n, a(n) = K*a(n-2) -(n-2)/(n-5). - _Washington Bomfim_, Aug 05 2008

%C Equals (1,2,3,4,...) convolved with (1,0,3,0,5,...). - _Gary W. Adamson_, Feb 16 2009

%C Equals row sums of triangle A177878.

%C Equals (1/2)*((1, 4, 10, 20, 35, 56, ...) + (1, 0, 2 0, 3, 0, 4, ...)).

%C Comment from _Ctibor O. Zizka_, Nov 21 2014: (Start)

%C With offset 4, a(n) is the number of different patterns of the 2-color 4-partition of n.

%C P(n)_(k;t) gives the number of different patterns of the t-color, k-partition of n.

%C P(n)_(k;t) = 1 + Sum(i=2..n) Sum(j=2..i) Sum(r=1..m) c_(i,j)*v_r*F_r(X_1,...,X_i).

%C P(n;i;j) = Sum(r=1..m) c_(i,j)*v_r*F_r(X_1,...,X_i).

%C m partition number of i.

%C c_(i,j) number of different coloring patterns on the r-th form (X_1,...,X_i) of i-partition with j-colors.

%C v_r number of i-partitions of n of the r-th form (X_1,...,X_i).

%C F_r(X_1,...,X_i) number of different patterns of the r-th form i-partition of n.

%C Some simple results:

%C P(1)_(k;t)=1, P(2)_(k;t)=2, P(3)_(k;t)=4, P(4)_(k;t)=11, etc.

%C P(n;1;1) = P(n;n;n) = 1 for all n;

%C P(n;2;2) = floor(n/2) (A004526);

%C P(n;3;2) = (n*n - 2*n + n mod 2)/4 (A002620).

%C This sequence is a(n) = P(n;4;2).

%C 2-coloring of 4-partition is (A,B,A,B) or (B,A,B,A).

%C Each 4-partition of n has one of the form (X_1,X_1,X_1,X_1),(X_1,X_1,X_1,X_2), (X_1,X_1,X_2,X_2),(X_1,X_1,X_2,X_3),(X_1,X_2,X_3,X_4).

%C The number of forms is m=5 which is the partition number of k=4.

%C Partition form (X_1,X_1,X_1,X_1) gives 1 pattern ((X_1A,X_1B,X_1A,X_1B), (X_1,X_1,X_1,X_2) gives 2 patterns, (X_1,X_1,X_2,X_2) gives 4 patterns, (X_1,X_1,X_2,X_3) gives 6 patterns and (X_1,X_2,X_3,X_4) gives 12 patterns.

%C Thus

%C a(n) = P(n;4;2) = 1*1*v_1 + 1*2*v_2 + 1*4*v_3 + 1*6*v_4 + 1*12*v_5

%C where v_r is the number of different 4-partitions of the r-th form (X_1,X_2,X_3,X_4) for a given n.

%C Example:

%C The 4-partitions of 8 are (2,2,2,2), (1,1,1,5), (1,1,3,3), (1,1,2,4), and (1,2,2,3):

%C (2,2,2,2) 1 pattern

%C (1,1,1,5), (1,1,5,1) 2 patterns

%C (1,1,3,3), (1,3,3,1), (3,1,1,3), (1,3,1,3) 4 patterns

%C (1,1,2,4), (1,1,4,2), (1,2,1,4), (1,2,4,1), (1,4,1,2), (2,1,1,4) 6 patterns

%C (2,2,1,3), (2,2,3,1), (2,1,2,3), (2,1,3,2), (2,3,2,1), (1,2,2,3) 6 patterns

%C Thus a(8) = P(8,4,2) = 1 + 2 + 4 + 6 + 6 = 19.

%C (End)

%C a(n) = length of run n+2 of consecutive 1's in A254338. - _Reinhard Zumkeller_, Feb 27 2015

%C Take a chessboard of (n+2) X (n+2) unit squares in which the a1 square is black. a(n) is the number of composite squares having black unit squares on their vertices. - _Ivan N. Ianakiev_, Jul 19 2018

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%D L. Smith, Polynomial Invariants of Finite Groups, A K Peters, 1995, p. 96.

%H T. D. Noe, <a href="/A005993/b005993.txt">Table of n, a(n) for n = 0..1000</a>

%H M. Benoumhani, M. Kolli, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL13/Benoumhani/benoumhani6.html">Finite topologies and partitions</a>, JIS 13 (2010) # 10.3.5, Lemma 6 3rd ine.

%H Washington Bomfim, <a href="http://commons.wikimedia.org/wiki/Image:Comentario1Blue3Reds.PNG">The 19 bracelets with 8 beads - one blue, three reds and four blacks.</a> [From _Washington Bomfim_, Aug 05 2008]

%H T. M. Brown <a href="https://arxiv.org/abs/1810.08235">On the unimodality of convolutions of sequences of binomial coefficients</a>, arXiv:1810.08235 [math.CO] (2018).

%H Johann Cigler, <a href="https://arxiv.org/abs/1711.03340">Some remarks on Rogers-Szegö polynomials and Losanitsch's triangle</a>, arXiv:1711.03340 [math.CO], 2017.

%H Dragomir Z. Djokovic, <a href="http://arXiv.org/abs/math.AC/0609262">Poincaré series [or Poincare series] of some pure and mixed trace algebras of two generic matrices</a>, arXiv:math/0609262 [math.AC], 2006. See Table 8.

%H Naihuan Jing, Kailash Misra, Carla Savage, <a href="http://arxiv.org/abs/math/9907183">On multi-color partitions and the generalized Rogers-Ramanujan identities</a>, arXiv:math/9907183 [math.CO], 1999.

%H S. M. Losanitsch, <a href="http://dx.doi.org/10.1002/cber.189703002144">Die Isomerie-Arten bei den Homologen der Paraffin-Reihe</a>, Chem. Ber. 30 (1897), 1917-1926.

%H S. M. Losanitsch, <a href="/A000602/a000602_1.pdf">Die Isomerie-Arten bei den Homologen der Paraffin-Reihe</a>, Chem. Ber. 30 (1897), 1917-1926. (Annotated scanned copy)

%H N. J. A. Sloane, <a href="/classic.html#LOSS">Classic Sequences</a>

%H L. Smith, <a href="http://dx.doi.org/10.1090/S0273-0979-97-00724-6">Polynomial invariants of finite groups. A survey of recent developments</a>. Bull. Amer. Math. Soc. (N.S.) 34 (1997), no. 3, 211-250. See page 218. MR1433171 (98i:13009).

%H <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (2,1,-4,1,2,-1).

%F l(c, r) = 1/2 C(c+r-3, r) + 1/2 d(c, r), where d(c, r) is C((c + r - 3)/2, r/2) if c is odd and r is even, 0 if c is even and r is odd, C((c + r - 4)/2, r/2) if c is even and r is even, C((c + r - 4)/2, (r - 1)/2) if c is odd and r is odd.

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

%F a(2n) = (n+1)(2n^2+4n+3)/3, a(2n+1) = (n+1)(n+2)(2n+3)/3. a(-4-n) = -a(n).

%F From _Yosu Yurramendi_, Sep 12 2008: (Start)

%F a(n+1) = a(n) + A008794(n+3) with a(1)=1.

%F a(n) = A027656(n) + 2*A006918(n).

%F a(n+2) = a(n) + A000982(n+2) with a(1)=1, a(2)=2. (End)

%F a(n) = 2*a(n-1) + a(n-2) - 4*a(n-3) + a(n-4) + 2*a(n-5) - a(n-6). - _Jaume Oliver Lafont_, Dec 05 2008

%F a(n) = (n^3 + 6*n^2 + 11*n + 6)/12 + ((n+2)/4)[n even] (the bracket means that the second term is added if and only if n is even). - _Benoit Jubin_, Mar 31 2012

%F a(n) = (1/12)*n*(n+1)*(n+2) + (1/4)*(n+1)*(1/2)*(1-(-1)^n), with offset 1. - _Yosu Yurramendi_, Jun 20 2013

%F a(n) = Sum_{i=0..n+1} ceiling(i/2) * round(i/2) = Sum_{i=0..n+2} floor(i/2)^2. - _Bruno Berselli_, Aug 30 2013

%F a(n) = (n + 2)*(3*(-1)^n + 2*n^2 + 8*n + 9)/24. - _Ilya Gutkovskiy_, May 04 2016

%F Recurrence formula: a(n) = ((n+2)*a(n-2)+2*a(n-1)-n)/(n-2), a(1)=1, a(2)=2. - _Gerry Martens_, Jun 10 2018

%e a(2) = 6, since ( x1*y1, x2*y2, x1*x1+y1*y1, x2*x2+y2*y2, x1*x2+y1*y2, x1*y2+x2*y1 ) are a basis for homogeneous quadratic invariant polynomials.

%p g := proc(n) local i; add(floor(i/2)^2,i=1..n+1) end: # Joseph S. Riel (joer(AT)k-online.com), Mar 22 2002

%p a:= n-> (Matrix([[1, 0\$3, -1, -2]]).Matrix(6, (i,j)-> if (i=j-1) then 1 elif j=1 then [2, 1, -4, 1, 2, -1][i] else 0 fi)^n)[1,1]; seq (a(n), n=0..44); # _Alois P. Heinz_, Jul 31 2008

%t CoefficientList[Series[(1+x^2)/((1-x)^2*(1-x^2)^2),{x,0,44}],x] (* _Jean-François Alcover_, Apr 08 2011 *)

%t LinearRecurrence[{2,1,-4,1,2,-1},{1,2,6,10,19,28},50] (* _Harvey P. Dale_, Feb 20 2012 *)

%o import Data.List (inits, intersperse)

%o a005993 n = a005994_list !! n

%o a005993_list = map (sum . zipWith (*) (intersperse 0 [1, 3 ..]) . reverse) \$

%o tail \$ inits [1..]

%o -- _Reinhard Zumkeller_, Feb 27 2015

%o (MAGMA) I:=[1,2,6,10,19,28]; [n le 6 select I[n] else 2*Self(n-1)+Self(n-2)-4*Self(n-3)+Self(n-4)+2*Self(n-5)-Self(n-6): n in [1..60]]; // _Vincenzo Librandi_, Jul 19 2015

%o (PARI) a(n)=polcoeff((1+x^2)/(1-x)^2/(1-x^2)^2+x*O(x^n),n)

%o (PARI) a(n) = (binomial(n+3, n) + (1-n%2)*binomial((n+2)/2, n>>1))/2 \\ _Washington Bomfim_, Aug 05 2008

%o (PARI) a = vector(50); a[1]=1; a[2]=2;

%o for(n=3, 50, a[n] = ((n+2)*a[n-2]+2*a[n-1]-n)/(n-2)); a \\ _Gerry Martens_, Jun 03 2018

%o (Sage)

%o def A005993():

%o a, b, to_be = 0, 0, True

%o while True:

%o yield (a*(a*(2*a+9)+13)+b*(b+1)*(2*b+1)+6)//6

%o if to_be: b += 1

%o else: a += 1

%o to_be = not to_be

%o a = A005993()

%o [a.next() for _ in range(48)] # _Peter Luschny_, May 04 2016

%Y Cf. A177878.

%Y Partial sums of A008794 (without 0). - _Bruno Berselli_, Aug 30 2013

%Y Cf. A254338, A002260, A005408, A282011.

%K nonn,easy,nice

%O 0,2

%A _N. J. A. Sloane_, Winston C. Yang (yang(AT)math.wisc.edu)

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Last modified November 13 00:03 EST 2018. Contains 317118 sequences. (Running on oeis4.)