A272232 - cp's OEIS frontend

A272232 Smallest k > 0 such that R_k//n//R_k is prime, where R_k is the repunit A002275(k) of length k and // denotes concatenation; or -1 if no such k exists.

n	a(n)
0	1
1	9
2	-1
3	1
4	2
5	1
6	10
7	3
8	1
9	1
10	3
11	-1
12	2
13	3
14	33
15	1
16	2
17	1
18	1
19	21
20	1
21	2
22	-1
23	1
24	7
25	48
26	292
27	4
28	3
29	1
30	1
31	2
32	1
33	-1
34	135
35	-1
36	1
37	-1
38	1
39	34
40	3
41	3
42	40
43	2
44	-1
45	1
46	3
47	1
48	1
49	32
50	61
51	1
52	2
53	1
54	137
55	-1
56	3
57	1
58	2
59	42
60	1
61	14
62	1
63	262
64	2
65	22
66	-1
67	3
68	9
69	2
70	33
71	73
72	1
73	3
74	1
75	2
76	3
77	-1
78	2
79	2
80	1

[1, 9, -1, 1, 2, 1, 10, 3, 1, 1, 3, -1, 2, 3, 33, 1, 2, 1, 1, 21, 1, 2, -1, 1, 7, 48, 292, 4, 3, 1, 1, 2, 1, -1, 135, -1, 1, -1, 1, 34, 3, 3, 40, 2, -1, 1, 3, 1, 1, 32, 61, 1, 2, 1, 137, -1, 3, 1, 2, 42, 1, 14, 1, 262, 2, 22, -1, 3, 9, 2, 33, 73, 1, 3, 1, 2, 3, -1, 2, 2, 1]

cp's OEIS Frontend

A272232 Smallest k > 0 such that R_k//n//R_k is prime, where R_k is the repunit A002275(k) of length k and // denotes concatenation; or -1 if no such k exists.

Table of values

List of values

n	a(n)
0	1
1	9
2	-1
3	1
4	2
5	1
6	10
7	3
8	1
9	1
10	3
11	-1
12	2
13	3
14	33
15	1
16	2
17	1
18	1
19	21
20	1
21	2
22	-1
23	1
24	7
25	48
26	292
27	4
28	3
29	1
30	1
31	2
32	1
33	-1
34	135
35	-1
36	1
37	-1
38	1
39	34
40	3
41	3
42	40
43	2
44	-1
45	1
46	3
47	1
48	1
49	32
50	61
51	1
52	2
53	1
54	137
55	-1
56	3
57	1
58	2
59	42
60	1
61	14
62	1
63	262
64	2
65	22
66	-1
67	3
68	9
69	2
70	33
71	73
72	1
73	3
74	1
75	2
76	3
77	-1
78	2
79	2
80	1

n	a(n)
0	1
1	9
2	-1
3	1
4	2
5	1
6	10
7	3
8	1
9	1
10	3
11	-1
12	2
13	3
14	33
15	1
16	2
17	1
18	1
19	21
20	1
21	2
22	-1
23	1
24	7
25	48
26	292
27	4
28	3
29	1
30	1
31	2
32	1
33	-1
34	135
35	-1
36	1
37	-1
38	1
39	34
40	3
41	3
42	40
43	2
44	-1
45	1
46	3
47	1
48	1
49	32
50	61
51	1
52	2
53	1
54	137
55	-1
56	3
57	1
58	2
59	42
60	1
61	14
62	1
63	262
64	2
65	22
66	-1
67	3
68	9
69	2
70	33
71	73
72	1
73	3
74	1
75	2
76	3
77	-1
78	2
79	2
80	1

n	a(n)
0	1
1	9
2	-1
3	1
4	2
5	1
6	10
7	3
8	1
9	1
10	3
11	-1
12	2
13	3
14	33
15	1
16	2
17	1
18	1
19	21
20	1
21	2
22	-1
23	1
24	7
25	48
26	292
27	4
28	3
29	1
30	1
31	2
32	1
33	-1
34	135
35	-1
36	1
37	-1
38	1
39	34
40	3
41	3
42	40
43	2
44	-1
45	1
46	3
47	1
48	1
49	32
50	61
51	1
52	2
53	1
54	137
55	-1
56	3
57	1
58	2
59	42
60	1
61	14
62	1
63	262
64	2
65	22
66	-1
67	3
68	9
69	2
70	33
71	73
72	1
73	3
74	1
75	2
76	3
77	-1
78	2
79	2
80	1