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A007974
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Number of permutations that are n-3 "block reversals" away from 12...n.
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4
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OFFSET
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3,2
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LINKS
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FORMULA
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PROG
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(Python)
def a(n):
perm = tuple(range(1, n+1)); reach = {perm}; frontier = {perm}
for k in range(n-3):
r1 = set()
while len(frontier) > 0:
q = frontier.pop()
for i in range(n):
for j in range(i+1, n+1):
qp = list(q)
qp[i:j] = qp[i:j][::-1]
qp = tuple(qp)
if qp not in reach:
reach.add(qp)
r1.add(qp)
frontier = r1
return len(frontier)
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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