A309491 - cp's OEIS frontend

A309491 Let gcd_2(b,c) be the second-largest common divisor of non-coprime integers b and c; then a(n) = Sum_{k=1..n} gcd_2(k,n). If b and c are coprime, then gcd_2(b,c) = 0.

n	a(n)
1	0
2	1
3	1
4	3
5	1
6	6
7	1
8	8
9	5
10	10
11	1
12	17
13	1
14	14
15	11
16	20
17	1
18	26
19	1
20	29
21	15
22	22
23	1
24	44
25	9
26	26
27	21
28	41
29	1
30	56
31	1
32	48
33	23
34	34
35	17
36	73
37	1
38	38
39	27
40	76
41	1
42	80
43	1
44	65
45	51
46	46
47	1
48	108
49	13
50	74
51	35
52	77
53	1
54	102
55	25
56	108
57	39
58	58
59	1
60	157
61	1

[0, 1, 1, 3, 1, 6, 1, 8, 5, 10, 1, 17, 1, 14, 11, 20, 1, 26, 1, 29, 15, 22, 1, 44, 9, 26, 21, 41, 1, 56, 1, 48, 23, 34, 17, 73, 1, 38, 27, 76, 1, 80, 1, 65, 51, 46, 1, 108, 13, 74, 35, 77, 1, 102, 25, 108, 39, 58, 1, 157, 1]

cp's OEIS Frontend

A309491 Let gcd_2(b,c) be the second-largest common divisor of non-coprime integers b and c; then a(n) = Sum_{k=1..n} gcd_2(k,n). If b and c are coprime, then gcd_2(b,c) = 0.

Table of values

List of values

n	a(n)
1	0
2	1
3	1
4	3
5	1
6	6
7	1
8	8
9	5
10	10
11	1
12	17
13	1
14	14
15	11
16	20
17	1
18	26
19	1
20	29
21	15
22	22
23	1
24	44
25	9
26	26
27	21
28	41
29	1
30	56
31	1
32	48
33	23
34	34
35	17
36	73
37	1
38	38
39	27
40	76
41	1
42	80
43	1
44	65
45	51
46	46
47	1
48	108
49	13
50	74
51	35
52	77
53	1
54	102
55	25
56	108
57	39
58	58
59	1
60	157
61	1

n	a(n)
1	0
2	1
3	1
4	3
5	1
6	6
7	1
8	8
9	5
10	10
11	1
12	17
13	1
14	14
15	11
16	20
17	1
18	26
19	1
20	29
21	15
22	22
23	1
24	44
25	9
26	26
27	21
28	41
29	1
30	56
31	1
32	48
33	23
34	34
35	17
36	73
37	1
38	38
39	27
40	76
41	1
42	80
43	1
44	65
45	51
46	46
47	1
48	108
49	13
50	74
51	35
52	77
53	1
54	102
55	25
56	108
57	39
58	58
59	1
60	157
61	1

n	a(n)
1	0
2	1
3	1
4	3
5	1
6	6
7	1
8	8
9	5
10	10
11	1
12	17
13	1
14	14
15	11
16	20
17	1
18	26
19	1
20	29
21	15
22	22
23	1
24	44
25	9
26	26
27	21
28	41
29	1
30	56
31	1
32	48
33	23
34	34
35	17
36	73
37	1
38	38
39	27
40	76
41	1
42	80
43	1
44	65
45	51
46	46
47	1
48	108
49	13
50	74
51	35
52	77
53	1
54	102
55	25
56	108
57	39
58	58
59	1
60	157
61	1