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%I #17 Sep 20 2019 21:41:22
%S 1,1,3,14,81,538,3926,30694,252939,2172830,19304190,176343390,
%T 1649008456,15730575554,152663683494,1503962954930,15012865733351,
%U 151622652413194,1547365078534578,15939972379349178,165597452660771610,1733609081727968492
%N Number of ways a river can cross a road 2n times.
%C More precisely, number of ways that a river (or directed line) that starts in the southwest and flows east can cross an east-west road 2n times (bisection of A005316).
%C Also number of ways a loop can cross two parallel roads 2n times. Some portion of loop must lie below lower road.
%H Jean-François Alcover, <a href="/A077054/b077054.txt">Table of n, a(n) for n = 0..27</a>
%F a(n) = A005316(2*n).
%t A005316 = Cases[Import["https://oeis.org/A005316/b005316.txt", "Table"], {_, _}][[All, 2]];
%t a[n_] := A005316[[2n+1]];
%t a /@ Range[0, 27] (* _Jean-François Alcover_, Sep 20 2019 *)
%Y Bisection of A005316. Cf. A005315, A085873, A086031.
%K nonn
%O 0,3
%A _N. J. A. Sloane_ and _Jon Wild_, Nov 29 2002
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