OFFSET
1,2
COMMENTS
From Nathaniel Johnston, Sep 30 2011: (Start)
A k-digit number d_1 d_2 ... d_k is in this sequence if there is a multigraph with k vertices (representing the k digits) with the properties that: (1) there are at most two edges connecting d_i and d_{i+1}, and (2) there are no edges connecting d_i and d_j if i and j are not consecutive integers. In the title, "nontrivial" means that this multigraph must be connected (which eliminates terms like 1111 and 1122).
For k > 1, there are 2^k-2 = A000918(k) terms in this sequence with k digits.
(End)
LINKS
Nathaniel Johnston, Table of n, a(n) for n = 1..10000
Wikipedia, Hashiwokakero
MAPLE
lim:=5: L[0]:={0}: for n from 0 to lim do L[n+1]:={0, op(map(`+`, L[n], 11*10^n)), op(map(`+`, L[n], 22*10^n))}: od: `union`(seq(L[k], k=0..lim)); # Nathaniel Johnston, Sep 30 2011
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Edgar Almeida Ribeiro (edgar.a.ribeiro(AT)gmail.com), Sep 06 2008
STATUS
approved