OFFSET
1,4
COMMENTS
Papaya words are defined as palindromes or concatenations of two palindromes.
LINKS
Tanya Khovanova, Papaya Words and Numbers
EXAMPLE
The only three-character non-papaya pattern is abc - words with all distinct letters. Four-character non-papaya patterns are: aabc, abbc, abcc, abca, abcd.
MATHEMATICA
R[k_?EvenQ] := (1/2)*k*(BellB[1 + k/2] + BellB[k/2]);
R[k_?OddQ] := k*BellB[1 + (k - 1)/2];
b[0] = 1; b[n_] := b[n] = R[n] - Sum[EulerPhi[n/d]*b[d], {d, Most[ Divisors[n]]}];
a[n_] := BellB[n] - b[n];
Array[a, 25] (* Jean-François Alcover, Jul 02 2018, after Andrew Howroyd *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Tanya Khovanova, Sep 22 2009
EXTENSIONS
a(7)-a(25) from Andrew Howroyd, Mar 29 2016
STATUS
approved