A291777 - cp's OEIS frontend

A291777 a(n) = number of steps to reach a prime when x -> sigma(x)-1 is repeatedly applied to 2^n-1, or -1 if no prime is ever reached.

n	a(n)
2	0
3	0
4	1
5	0
6	1
7	0
8	1
9	3
10	2
11	9
12	2
13	0
14	7
15	3
16	4
17	0
18	2
19	0
20	1
21	4
22	1
23	4
24	2
25	3
26	4
27	2
28	12
29	22
30	8
31	0
32	3
33	3
34	4
35	3
36	1
37	2
38	2
39	3
40	3
41	4
42	3
43	13
44	2
45	16
46	3
47	8
48	3
49	14
50	17
51	9
52	37
53	4
54	7
55	4
56	7
57	11
58	4
59	3
60	14
61	0
62	14
63	8
64	1
65	6
66	8
67	73
68	26
69	10
70	1
71	32
72	6
73	10
74	2
75	6
76	2
77	33
78	2
79	4
80	52
81	12
82	16

[0, 0, 1, 0, 1, 0, 1, 3, 2, 9, 2, 0, 7, 3, 4, 0, 2, 0, 1, 4, 1, 4, 2, 3, 4, 2, 12, 22, 8, 0, 3, 3, 4, 3, 1, 2, 2, 3, 3, 4, 3, 13, 2, 16, 3, 8, 3, 14, 17, 9, 37, 4, 7, 4, 7, 11, 4, 3, 14, 0, 14, 8, 1, 6, 8, 73, 26, 10, 1, 32, 6, 10, 2, 6, 2, 33, 2, 4, 52, 12, 16]

cp's OEIS Frontend

A291777 a(n) = number of steps to reach a prime when x -> sigma(x)-1 is repeatedly applied to 2^n-1, or -1 if no prime is ever reached.

Table of values

List of values

n	a(n)
2	0
3	0
4	1
5	0
6	1
7	0
8	1
9	3
10	2
11	9
12	2
13	0
14	7
15	3
16	4
17	0
18	2
19	0
20	1
21	4
22	1
23	4
24	2
25	3
26	4
27	2
28	12
29	22
30	8
31	0
32	3
33	3
34	4
35	3
36	1
37	2
38	2
39	3
40	3
41	4
42	3
43	13
44	2
45	16
46	3
47	8
48	3
49	14
50	17
51	9
52	37
53	4
54	7
55	4
56	7
57	11
58	4
59	3
60	14
61	0
62	14
63	8
64	1
65	6
66	8
67	73
68	26
69	10
70	1
71	32
72	6
73	10
74	2
75	6
76	2
77	33
78	2
79	4
80	52
81	12
82	16

n	a(n)
2	0
3	0
4	1
5	0
6	1
7	0
8	1
9	3
10	2
11	9
12	2
13	0
14	7
15	3
16	4
17	0
18	2
19	0
20	1
21	4
22	1
23	4
24	2
25	3
26	4
27	2
28	12
29	22
30	8
31	0
32	3
33	3
34	4
35	3
36	1
37	2
38	2
39	3
40	3
41	4
42	3
43	13
44	2
45	16
46	3
47	8
48	3
49	14
50	17
51	9
52	37
53	4
54	7
55	4
56	7
57	11
58	4
59	3
60	14
61	0
62	14
63	8
64	1
65	6
66	8
67	73
68	26
69	10
70	1
71	32
72	6
73	10
74	2
75	6
76	2
77	33
78	2
79	4
80	52
81	12
82	16

n	a(n)
2	0
3	0
4	1
5	0
6	1
7	0
8	1
9	3
10	2
11	9
12	2
13	0
14	7
15	3
16	4
17	0
18	2
19	0
20	1
21	4
22	1
23	4
24	2
25	3
26	4
27	2
28	12
29	22
30	8
31	0
32	3
33	3
34	4
35	3
36	1
37	2
38	2
39	3
40	3
41	4
42	3
43	13
44	2
45	16
46	3
47	8
48	3
49	14
50	17
51	9
52	37
53	4
54	7
55	4
56	7
57	11
58	4
59	3
60	14
61	0
62	14
63	8
64	1
65	6
66	8
67	73
68	26
69	10
70	1
71	32
72	6
73	10
74	2
75	6
76	2
77	33
78	2
79	4
80	52
81	12
82	16