OFFSET
0,3
EXAMPLE
a(0) = 1: the empty word.
a(1) = 1: 1.
a(2) = 2: 11, 12.
a(3) = 5: 111, 112, 121, 122, 123.
a(4) = 14: 1111, 1112, 1121, 1122, 1123, 1211, 1212, 1213, 1221, 1223, 1231, 1232, 1233, 1234.
a(5) = 46: 11111, 11112, 11121, ..., 12343, 12344, 12345.
a(6) = 165: 111111, 111112, 111121, ..., 123454, 123455, 123456.
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Apr 09 2014
STATUS
approved