%I #17 Nov 27 2020 02:07:53
%S 1,2,2,4,8,12,24,48,96,180,360,720,1440,2820,5640,11280,22560,44760,
%T 89520,179040,358080,713760,1427520,2855040,5710080,11403060,22806120,
%U 45612240,91224480,182321460,364642920,729285840,1458571680,2916160800,5832321600
%N Number of n-digit binary strings such that for all 2 <= k <= n, the string formed by the first k digits of the original string is composed of at least one-quarter 1's and one-quarter 0's.
%H Alois P. Heinz and Charles R Greathouse IV, <a href="/A268691/b268691.txt">Table of n, a(n) for n = 0..3323</a> (0-1000 from Heinz)
%F For all n > 2 not equivalent to 1 mod 4, a(n) = 2a(n-1).
%e For n=2, the strings are 01 and 10. For n=3, they are 010, 011, 100, 101.
%p b:= proc(n, i, j) option remember; `if`(n=0, 1, (t->
%p `if`(t>=2 and 4*j<t, 0, b(n-1, sort([j, i+1])[]))+
%p `if`(t>=2 and 4*i<t, 0, b(n-1, sort([i, j+1])[])))(i+j+1))
%p end:
%p a:= n-> b(n, 0$2):
%p seq(a(n), n=0..50); # _Alois P. Heinz_, Feb 11 2016
%t b[n_, i_, j_] := b[n, i, j] = If[n == 0, 1, Function[t,
%t If[t >= 2 && 4j < t, 0, b[n-1, Sequence @@ Sort[{j, i+1}]]]+
%t If[t >= 2 && 4i < t, 0, b[n-1, Sequence @@ Sort[{i, j+1}]]]][i+j+1]];
%t a[n_] := b[n, 0, 0];
%t a /@ Range[0, 50] (* _Jean-François Alcover_, Nov 27 2020, after _Alois P. Heinz_ *)
%o (PARI) a(n)=if(n<2,return(n+1));my(u,v=vector((3*n+1)\4),mx,mn);v[1]=2;for(i=3,n,mn=(i+3)\4;mx=i-mn;u=vector(#v,j,if(j<mn||j>mx,0,if(j>1,v[j-1])+v[j]));v=u);vecsum(v) \\ _Charles R Greathouse IV_, Feb 11 2016
%o (PARI) first(n)=if(n<2,return(n+1));my(u,v=vector((3*n+1)\4),w=vector(n+1),mx,mn);w[1]=1;w[2]=w[3]=v[1]=2;for(i=3,n,mn=(i+3)\4;mx=i-mn;u=vector(#v,j,if(j<mn||j>mx,0,if(j>1,v[j-1])+v[j]));w[i+1]=vecsum(v=u));w \\ _Charles R Greathouse IV_, Feb 11 2016
%K nonn
%O 0,2
%A _Josh Speckman_, Feb 11 2016
%E More terms from _Alois P. Heinz_, Feb 11 2016