Under OEIS A000085, a Telephone number - also known as Involution number - is counting the number
of connection patterns in a telephone system with
n subscribers, or in a more mathematical term, the number of self-inverse permutations on
n letters. First 6 Telephone numbers are 1, 1, 2, 4, 10, 26,
the number of first
a vector of length
n containing first entries from the sequence.
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