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 A164392 Number of binary strings of length n with no substrings equal to 0001 or 0010. 7
 1, 2, 4, 8, 14, 25, 44, 78, 137, 241, 423, 743, 1304, 2289, 4017, 7050, 12372, 21712, 38102, 66865, 117340, 205918, 361361, 634145, 1112847, 1952911, 3427120, 6014177, 10554145, 18521234, 32502500, 57037912, 100094558, 175653705, 308250764, 540942382 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Nonnegative walks with n steps on the x-axis starting at the origin using steps {1,0,-1} and visiting no point more than twice. Note: a 0 step counts as a visit and a step but does not contribute to the length of the walk. - David Scambler, May 22 2012 LINKS Alois P. Heinz, Table of n, a(n) for n = 0..2000 (first 500 terms from R. H. Hardin) Index entries for linear recurrences with constant coefficients, signature (2, 0, -1, 1, -1). FORMULA From David Scambler, May 22 2012: (Start) G.f.: (1+x^3-x^4)/(1-2*x+x^3-x^4+x^5). a(n) = 2^n for n<4; otherwise, a(n) = a(n-1)+a(n-2)+a(n-4)+1. (End) MATHEMATICA CoefficientList[Series[ (1+x^3-x^4)/(1-2*x+x^3-x^4+x^5) , {x, 0, 45}], x] (* David Scambler, May 22 2012 *) PROG (PARI) x='x+O('x^50); Vec( (1+x^3-x^4)/(1-2*x+x^3-x^4+x^5) ) \\ G. C. Greubel, sep 18 2017 CROSSREFS Cf. A212584, A212585, A212586, A212587, A212589. Sequence in context: A164391 A164153 A212588 * A164152 A164390 A164151 Adjacent sequences:  A164389 A164390 A164391 * A164393 A164394 A164395 KEYWORD nonn,easy,walk AUTHOR R. H. Hardin, Aug 14 2009 EXTENSIONS Edited by Alois P. Heinz, Oct 27 2017 STATUS approved

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