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A368189 Number of (undirected) cycles in the n-Pell graph. 1
0, 1, 65, 4983940 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,3
LINKS
Eric Weisstein's World of Mathematics, Graph Cycle.
Eric Weisstein's World of Mathematics, Pell Graph.
PROG
(Python)
from itertools import product, combinations, groupby, islice
from networkx import empty_graph, simple_cycles
def A368189(n):
if n<=1 : return 0
G = empty_graph(v for v in product((0, 1, 2), repeat=n) if not any(len(list(g))&1 and k==2 for k, g in groupby(v)))
for a, b in combinations(G, 2):
r = tuple(islice((i for i in range(n) if a[i]!=b[i]), 3))
if len(r)==1 and a[r[0]]!=2 and b[r[0]]!=2:
G.add_edge(a, b)
elif len(r)==2 and r[0]+1==r[1] and a[r[0]] and b[r[0]] and a[r[0]]==a[r[1]] and b[r[0]]==b[r[1]]:
G.add_edge(a, b)
return sum(1 for c in simple_cycles(G)) # Chai Wah Wu, Jan 18 2024
CROSSREFS
Sequence in context: A238612 A185823 A035519 * A238842 A059755 A215657
KEYWORD
nonn,more
AUTHOR
Eric W. Weisstein, Dec 16 2023
STATUS
approved

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Last modified July 17 18:23 EDT 2024. Contains 374377 sequences. (Running on oeis4.)