0,4

Comments from David W. Wilson, Mar 10 2005: "Let M(n) be a map of n regions in a row. The number of ways to color M(n) if same-color regions are allowed to touch is given by A000110(n).

"For example, M(4) has A000110(4) = 15 such colorings: aaaa aaab aaba aabb aabc abaa abab abac abba abbb abbc abca abcb abcc abcd

"The number of colorings of M(n) that are equivalent to their reverse is given by A080107(n). For example, M(4) has A080107(4) = 7 colorings that are equivalent to their reversal: aaaa aabb abab abba abbc abca abcd

"The number of distinct colorings when reversals are counted as equivalent is given by ((A000110(n) + A080107(n))/2, which is essentially the present sequence. M(4) has 11 colorings that are distinct up to reversal: aaaa aaab aaba aabb aabc abab abac abba abbc abca abcd

"We can redo the whole analysis, this time forbidding same-color regions to touch. When we do, we get the same sequences, each with an extra 1 at the beginning."

Note that A056325 gives number of reversible string structures with n beads using a maximum of six different colors. .. and of course, any limit on the number of colors will be the same as this sequence above up to that number.

If the two ends of the line are distinguishable, so that 'abcb' and 'abac' are distinct, we get the Bell numbers, A000110(n - 1)

Comment from David Callan, Oct 10 2005: With a different offset, number of set partitions of [n] up to reflection (i<->n+1-i). E.g. there are 4 partitions of [3]: 123, 1-23, 13-2, 1-2-3 but not 12-3 because it is the reflection of 1-23.

For n=4, possible arrangements are 'abab', 'abac', 'abca', 'abcd'; we do not include 'abcb' since it is equivalent to 'abac' (if you reverse and renormalize)

with (combinat): b:= n-> coeff (series (exp ((exp(2*x)-3)/2 +exp(x)), x, n+1), x, n)*n!: a:= n-> `if`(n=0, 1, (bell(n-1) + `if`(modp(n, 2)=1, b((n-1)/2), add (binomial (n/2-1, k) *b(k), k=0..n/2-1)))/2): seq (a(n), n=0..23); [From Alois P. Heinz (heinz(AT)hs-heilbronn.de), Sep 05 2008]

Cf. A000110, A056325.

Sequence in context: A124504 A056324 A056325 this_sequence A123418 A123412 A074408

Adjacent sequences: A103290 A103291 A103292 this_sequence A103294 A103295 A103296

nonn

hv(AT)crypt.org (Hugo van der Sanden), Mar 10 2005

More terms from David W. Wilson, Mar 10 2005