A306861 - cp's OEIS frontend

A306861 The concatenation kpk is the number obtained by placing k leading and trailing 1's around the prime p; a(n) is the smallest k such that kpk is prime, where p = prime(n), or -1 if no such k exists.

n	a(n)
1	-1
2	1
3	1
4	3
5	-1
6	3
7	1
8	21
9	1
10	1
11	2
12	-1
13	3
14	2
15	1
16	1
17	42
18	14
19	3
20	73
21	3
22	2
23	1
24	4
25	3
26	-1
27	2
28	1
29	3
30	1
31	3
32	1
33	3
34	3
35	1
36	6
37	2
38	3
39	192
40	1
41	4
42	3
43	3
44	8
45	1
46	9
47	36
48	5
49	12
50	5
51	18
52	1
53	26
54	1
55	16
56	10
57	15
58	2
59	72
60	22
61	3
62	4
63	2
64	4
65	5
66	1
67	12
68	5
69	13
70	3
71	9
72	1
73	6
74	60
75	2
76	1
77	58

[-1, 1, 1, 3, -1, 3, 1, 21, 1, 1, 2, -1, 3, 2, 1, 1, 42, 14, 3, 73, 3, 2, 1, 4, 3, -1, 2, 1, 3, 1, 3, 1, 3, 3, 1, 6, 2, 3, 192, 1, 4, 3, 3, 8, 1, 9, 36, 5, 12, 5, 18, 1, 26, 1, 16, 10, 15, 2, 72, 22, 3, 4, 2, 4, 5, 1, 12, 5, 13, 3, 9, 1, 6, 60, 2, 1, 58]

cp's OEIS Frontend

A306861 The concatenation kpk is the number obtained by placing k leading and trailing 1's around the prime p; a(n) is the smallest k such that kpk is prime, where p = prime(n), or -1 if no such k exists.

Table of values

List of values

n	a(n)
1	-1
2	1
3	1
4	3
5	-1
6	3
7	1
8	21
9	1
10	1
11	2
12	-1
13	3
14	2
15	1
16	1
17	42
18	14
19	3
20	73
21	3
22	2
23	1
24	4
25	3
26	-1
27	2
28	1
29	3
30	1
31	3
32	1
33	3
34	3
35	1
36	6
37	2
38	3
39	192
40	1
41	4
42	3
43	3
44	8
45	1
46	9
47	36
48	5
49	12
50	5
51	18
52	1
53	26
54	1
55	16
56	10
57	15
58	2
59	72
60	22
61	3
62	4
63	2
64	4
65	5
66	1
67	12
68	5
69	13
70	3
71	9
72	1
73	6
74	60
75	2
76	1
77	58

n	a(n)
1	-1
2	1
3	1
4	3
5	-1
6	3
7	1
8	21
9	1
10	1
11	2
12	-1
13	3
14	2
15	1
16	1
17	42
18	14
19	3
20	73
21	3
22	2
23	1
24	4
25	3
26	-1
27	2
28	1
29	3
30	1
31	3
32	1
33	3
34	3
35	1
36	6
37	2
38	3
39	192
40	1
41	4
42	3
43	3
44	8
45	1
46	9
47	36
48	5
49	12
50	5
51	18
52	1
53	26
54	1
55	16
56	10
57	15
58	2
59	72
60	22
61	3
62	4
63	2
64	4
65	5
66	1
67	12
68	5
69	13
70	3
71	9
72	1
73	6
74	60
75	2
76	1
77	58

n	a(n)
1	-1
2	1
3	1
4	3
5	-1
6	3
7	1
8	21
9	1
10	1
11	2
12	-1
13	3
14	2
15	1
16	1
17	42
18	14
19	3
20	73
21	3
22	2
23	1
24	4
25	3
26	-1
27	2
28	1
29	3
30	1
31	3
32	1
33	3
34	3
35	1
36	6
37	2
38	3
39	192
40	1
41	4
42	3
43	3
44	8
45	1
46	9
47	36
48	5
49	12
50	5
51	18
52	1
53	26
54	1
55	16
56	10
57	15
58	2
59	72
60	22
61	3
62	4
63	2
64	4
65	5
66	1
67	12
68	5
69	13
70	3
71	9
72	1
73	6
74	60
75	2
76	1
77	58