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A137720 as a simple table

na(n)
01
11
21
3-1
4-13
5-67
6-285
7-1119
8-4215
9-15505
10-56239
11-202309
12-724499
13-2589521
14-9254363
15-33111969
16-118725597
17-426892131
18-1539965973
19-5575175319
20-20260052337
21-73908397851
22-270657727593
23-994938310059

[1,1,1,-1,-13,-67,-285,-1119,-4215,-15505,-56239,
 -202309,-724499,-2589521,-9254363,-33111969,
 -118725597,-426892131,-1539965973,-5575175319,
 -20260052337,-73908397851,-270657727593,
 -994938310059]