A338366 - cp's OEIS frontend

A338366 a(n) = smallest positive number k with all digits equal such that the concatenation k||n||k is prime, or -1 if no such k exists.

n	a(n)
0	1
1	3
2	7
3	1
4	11
5	1
6	7777
7	3
8	1
9	1
10	9
11	-1
12	7
13	33
14	99
15	1
16	3
17	1
18	1
19	9
20	1
21	11
22	-1
23	1
24	7
25	3
26	7777777777
27	1111
28	111
29	1
30	1
31	3
32	1
33	-1
34	3
35	33
36	1
37	3
38	1
39	77777777777777
40	111
41	3
42	1111111111111111111111111111111111111111
43	3
44	-1
45	1
46	3
47	1
48	1
49	999
50	7
51	1
52	11
53	1
54	7
55	-1
56	33
57	1
58	3
59	3
60	1
61	3
62	1

[1, 3, 7, 1, 11, 1, 7777, 3, 1, 1, 9, -1, 7, 33, 99, 1, 3, 1, 1, 9, 1, 11, -1, 1, 7, 3, 7777777777, 1111, 111, 1, 1, 3, 1, -1, 3, 33, 1, 3, 1, 77777777777777, 111, 3, 1111111111111111111111111111111111111111, 3, -1, 1, 3, 1, 1, 999, 7, 1, 11, 1, 7, -1, 33, 1, 3, 3, 1, 3, 1]

cp's OEIS Frontend

A338366 a(n) = smallest positive number k with all digits equal such that the concatenation k||n||k is prime, or -1 if no such k exists.

Table of values

List of values