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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.

A291776 a(n) = prime that is eventually reached when x -> sigma(x)-1 is repeatedly applied to 2^n-1, or -1 if no prime is ever reached.

Table of values

n a(n)
2 3
3 7
4 23
5 31
6 103
7 127
8 431
9 911
10 1847
11 6719
12 10487
13 8191
14 56999
15 41399
16 135647
17 131071
18 560159
19 524287
20 1999871
21 3982271
22 5909759
23 17512991
24 46092239
25 46335599
26 164460119
27 186592247
28 736727807
29 3926707199
30 4146049487
31 2147483647
32 8994904463
33 11132323439

List of values

[3, 7, 23, 31, 103, 127, 431, 911, 1847, 6719, 10487, 8191, 56999, 41399, 135647, 131071, 560159, 524287, 1999871, 3982271, 5909759, 17512991, 46092239, 46335599, 164460119, 186592247, 736727807, 3926707199, 4146049487, 2147483647, 8994904463, 11132323439]

Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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