Search: id:A077054
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%I A077054
%S A077054 1,1,3,14,81,538,3926,30694,252939,2172830,19304190,176343390,1649008456,
%T A077054 15730575554,152663683494,1503962954930,15012865733351,151622652413194,
%U A077054 1547365078534578,15939972379349178,165597452660771610,1733609081727968492
%N A077054 Number of ways a river can cross a road 2n times.
%C A077054 More precisely, number of ways that a river (or directed line) that starts 
               in the South-West and flows East can cross an East-West road 2n times 
               (bisection of A005316).
%C A077054 Also number of ways a loop can cross two parallel roads 2n times. Some 
               portion of loop must lie below lower road.
%Y A077054 Bisection of A005316. Cf. A005315, A085873, A086031.
%Y A077054 Sequence in context: A107596 A000264 A009053 this_sequence A032332 A032080 
               A020104
%Y A077054 Adjacent sequences: A077051 A077052 A077053 this_sequence A077055 A077056 
               A077057
%K A077054 nonn
%O A077054 0,3
%A A077054 N. J. A. Sloane (njas(AT)research.att.com) and Jon Wild (wild(AT)music.mcgill.ca), 
               Nov 29 2002

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