A377947 - cp's OEIS frontend

A377947 Unfriendly EKG sequence: a(1) = 1; a(2) = 3; for n > 2, a(n) = least number not already used which shares a factor with a(n-1) and is less than half or more than twice a(n-1).

n	a(n)
1	1
2	3
3	9
4	21
5	6
6	2
7	8
8	18
9	4
10	10
11	22
12	46
13	12
14	26
15	54
16	14
17	30
18	5
19	15
20	33
21	11
22	44
23	16
24	34
25	70
26	7
27	28
28	58
29	20
30	42
31	86
32	24
33	50
34	102
35	17
36	51
37	105
38	25
39	55
40	115
41	23
42	69
43	27
44	57
45	19
46	76
47	32
48	66
49	134
50	36
51	74
52	150
53	35
54	75
55	153
56	39
57	13
58	52
59	106
60	38
61	78
62	158
63	40
64	82
65	166
66	48
67	98
68	198
69	45
70	93
71	31
72	124

[1, 3, 9, 21, 6, 2, 8, 18, 4, 10, 22, 46, 12, 26, 54, 14, 30, 5, 15, 33, 11, 44, 16, 34, 70, 7, 28, 58, 20, 42, 86, 24, 50, 102, 17, 51, 105, 25, 55, 115, 23, 69, 27, 57, 19, 76, 32, 66, 134, 36, 74, 150, 35, 75, 153, 39, 13, 52, 106, 38, 78, 158, 40, 82, 166, 48, 98, 198, 45, 93, 31, 124]

cp's OEIS Frontend

A377947 Unfriendly EKG sequence: a(1) = 1; a(2) = 3; for n > 2, a(n) = least number not already used which shares a factor with a(n-1) and is less than half or more than twice a(n-1).

Table of values

List of values

n	a(n)
1	1
2	3
3	9
4	21
5	6
6	2
7	8
8	18
9	4
10	10
11	22
12	46
13	12
14	26
15	54
16	14
17	30
18	5
19	15
20	33
21	11
22	44
23	16
24	34
25	70
26	7
27	28
28	58
29	20
30	42
31	86
32	24
33	50
34	102
35	17
36	51
37	105
38	25
39	55
40	115
41	23
42	69
43	27
44	57
45	19
46	76
47	32
48	66
49	134
50	36
51	74
52	150
53	35
54	75
55	153
56	39
57	13
58	52
59	106
60	38
61	78
62	158
63	40
64	82
65	166
66	48
67	98
68	198
69	45
70	93
71	31
72	124

n	a(n)
1	1
2	3
3	9
4	21
5	6
6	2
7	8
8	18
9	4
10	10
11	22
12	46
13	12
14	26
15	54
16	14
17	30
18	5
19	15
20	33
21	11
22	44
23	16
24	34
25	70
26	7
27	28
28	58
29	20
30	42
31	86
32	24
33	50
34	102
35	17
36	51
37	105
38	25
39	55
40	115
41	23
42	69
43	27
44	57
45	19
46	76
47	32
48	66
49	134
50	36
51	74
52	150
53	35
54	75
55	153
56	39
57	13
58	52
59	106
60	38
61	78
62	158
63	40
64	82
65	166
66	48
67	98
68	198
69	45
70	93
71	31
72	124

n	a(n)
1	1
2	3
3	9
4	21
5	6
6	2
7	8
8	18
9	4
10	10
11	22
12	46
13	12
14	26
15	54
16	14
17	30
18	5
19	15
20	33
21	11
22	44
23	16
24	34
25	70
26	7
27	28
28	58
29	20
30	42
31	86
32	24
33	50
34	102
35	17
36	51
37	105
38	25
39	55
40	115
41	23
42	69
43	27
44	57
45	19
46	76
47	32
48	66
49	134
50	36
51	74
52	150
53	35
54	75
55	153
56	39
57	13
58	52
59	106
60	38
61	78
62	158
63	40
64	82
65	166
66	48
67	98
68	198
69	45
70	93
71	31
72	124