A242206 Number of length n binary words which contain 00 and 01 and 10 and 11 as (possibly overlapping) contiguous subsequences.
4, 18, 54, 138, 324, 724, 1568, 3326, 6954, 14390, 29552, 60344, 122684, 248586, 502366, 1013122, 2039804, 4101532, 8238520, 16534390, 33161554, 66473198, 133189224, 266771328, 534178324, 1069385154, 2140434438, 4283561466, 8571479604, 17150008420, 34311422672
Offset: 5
Keywords
Examples
a(5) = 4 because we have: 00110, 01100, 10011, 11001.
Links
- Alois P. Heinz, Table of n, a(n) for n = 5..1000
- Eric Weisstein's World of Mathematics, Coin Tossing
Programs
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Mathematica
sol=Solve[{A==va (z^2+z A+z C),B==vb (z^2+z A+z C),C==vc (z^2+z B+z D), D==vd (z^2+z B+z D)}, {A,B,C,D}]; S=1/(1-2 z-A-B-C-D); vsub={va->ua-1,vb->ub-1,vc->uc-1,vd->ud-1}; Fz[z_,ua_,ub_,uc_,ud_]=Simplify[S/.sol/.vsub]; G[z_]=Simplify[Fz[z,1,1,1,0]+Fz[z,0,1,1,1]+Fz[z,1,0,1,1] +Fz[z,1,1,0,1] -Fz[z,1,1,0,0] -Fz[z,1,0,1,0]-Fz[z,1,0,0,1]-Fz[z,0,1,1,0] -Fz[z,0,1,0,1] -Fz[z,0,0,1,1]+Fz[z,1,0,0,0]+Fz[z,0,1,0,0] +Fz[z,0,0,1,0] +Fz[z,0,0,0,1] -Fz[z,0,0,0,0]]; Drop[Flatten[CoefficientList[Series[1/(1-2z)-G[z], {z,0,40}],z]],5] CoefficientList[Series[-2x^5(-2+x+2x^2)/((2x-1)(x^2+x-1)(x-1)^2),{x,0,50}],x] (* Harvey P. Dale, May 30 2018 *)
Formula
G.f.: -2*x^5*(-2+x+2*x^2)/((2*x-1)*(x^2+x-1)*(x-1)^2). - Alois P. Heinz, May 07 2014
Comments