Permutations Restricted By Primes
Leroy Quet
qq-quet at mindspring.com
Sun Aug 1 15:26:03 CEST 2004
I was wondering, how many permutations <n(1),n(2),...,n(m)> are there of
<1,2,...,m>, where
n(k) < n(k-1) whenever k is prime,
n(k) > n(k-1) whenever k is composite?
For example, for m = 4 we have the permutations:
3, 2, 1, 4
4, 3, 1, 2
4, 2, 1, 3
So the 4th term of the sequence giving the number of permutations would
be 3.
I do not know if this sequence is in the EIS.
Another possible sequence would be the number of permutations, where (as
above)
n(k) < n(k-1) whenever k is prime,
but there is no restriction on n(k) if k is composite.
And of course, we can use other criteria besides primeness to generate
other permutation-counting sequences using the same basic idea.
thanks,
Leroy Quet
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