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A003687 as a simple table

na(n)
01
12
21
3-1
4-3
5-1
6-7
7-1
8-43
9-1
10-1807
11-1
12-3263443
13-1
14-10650056950807
15-1
16-113423713055421844361000443
17-1
18-12864938683278671740537145998360961546653259485195807
19-1

[1,2,1,-1,-3,-1,-7,-1,-43,-1,-1807,-1,-3263443,-1,
 -10650056950807,-1,-113423713055421844361000443,
 -1,
 -1286493868327867174053714599836096154665325948519\
 5807,-1]