(Python)
# A function that calculates
A091299[n] from Janez Brank.
def CountGray(n):
def Recurse(unused, lastVal, nextSet):
count = 0
for changedBit in range(0, min(nextSet + 1, n)):
newVal = lastVal ^ (1 << changedBit)
mask = 1 << newVal
if unused & mask:
if unused == mask:
count += 1
else:
count += Recurse(
unused & ~mask, newVal, max(nextSet, changedBit + 1)
)
return count
count = Recurse((1 << (1 << n)) - 2, 0, 0)
for i in range(1, n + 1):
count *= 2 * i
return max(1, count)
[CountGray(n) for n in range(1, 4)]
