A281120 For n odd, a(n) is the least positive integer not already used. For n even, a(n) is the least positive integer not already used that is not coprime to a(n/2).

2, 4, 3, 6, 5, 9, 7, 8, 10, 15, 11, 12, 13, 14, 16, 18, 17, 20, 19, 21, 22, 33, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 34, 51, 35, 36, 37, 38, 39, 42, 40, 44, 41, 45, 43, 46, 47, 48, 49, 50, 52, 54, 53, 57, 55, 56, 58, 87, 59, 60, 61, 62, 63, 64, 65, 66, 67

Offset

1,1

Comments

In other words, gcd(a(n), a(2n)) > 1 for any n>0 (see A281121 for value of gcd(a(n), a(2n))).

This sequence cannot contain the value 1.

This sequence contains all values > 1.

This sequence is related to A082746: here we have gcd(a(n), a(2n)) > 1 and there we have gcd(a(n), a(2n)) = 1.

Links

Rémy Sigrist, Table of n, a(n) for n = 1..10000

Rémy Sigrist, PARI program for A281120

Example

Here is an illustration of the first terms:
n    a(n)  a(2n)  gcd(a(n),a(2n))
--   ----  -----  ---------------
1      2     4     2
2      4     6     2
3      3     9     3
4      6     8     2
5      5    15     5
6      9    12     3
7      7    14     7
8      8    18     2
9     10    20    10
10    15    21     3
11    11    33    11
12    12    24    12
13    13    26    13
14    14    28    14
15    16    30     2
16    18    32     2
17    17    51    17
18    20    36     4
19    19    38    19
20    21    42    21
21    22    44    22
22    33    45     3
23    23    46    23
24    24    48    24
25    25    50    25

Crossrefs

Cf. A082746, A281121.

Sequence in context: A354373 A354374 A354453 * A246682 A143691 A129767

Adjacent sequences: A281117 A281118 A281119 * A281121 A281122 A281123

Keyword

nonn

Author

Rémy Sigrist, Jan 15 2017

Status

approved