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This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A057004 Array T(n,k) = number of conjugacy classes of subgroups of index k in free group of rank n, read by antidiagonals.

Table of values

n a(n)
1 1
2 1
3 1
4 1
5 3
6 1
7 1
8 7
9 7
10 1
11 1
12 15
13 41
14 26
15 1
16 1
17 31
18 235
19 604
20 97
21 1
22 1
23 63
24 1361
25 14120
26 13753
27 624
28 1
29 1
30 127
31 7987
32 334576
33 1712845
34 504243
35 4163
36 1
37 1
38 255
39 47321
40 7987616
41 207009649
42 371515454
43 24824785
44 34470
45 1
46 1
47 511
48 281995
49 191318464

List of values

[1, 1, 1, 1, 3, 1, 1, 7, 7, 1, 1, 15, 41, 26, 1, 1, 31, 235, 604, 97, 1, 1, 63, 1361, 14120, 13753, 624, 1, 1, 127, 7987, 334576, 1712845, 504243, 4163, 1, 1, 255, 47321, 7987616, 207009649, 371515454, 24824785, 34470, 1, 1, 511, 281995, 191318464]