A122280 -id:A122280 - cp's OEIS frontend

A122462 a(n) = gcd(b(n-1),b(n)), where b(i) = A122280(i).

2, 2, 3, 3, 3, 2, 5, 5, 3, 2, 2, 7, 7, 3, 2, 11, 11, 3, 3, 2, 2, 13, 13, 3, 2, 17, 17, 3, 2, 5, 5, 7, 2, 19, 19, 3, 5, 2, 23, 23, 3, 2, 2, 2, 2, 11, 5, 2, 29, 29, 3, 7, 7, 2, 31, 31, 3, 2, 13, 5, 2, 2, 37, 37, 3, 3, 3, 2, 2, 41, 41, 3, 3, 7, 11, 2, 43, 43, 3, 2, 2, 47, 47, 3, 2, 2, 5, 5, 5, 2, 2, 2

Offset: 3

Klaus Brockhaus, Sep 09 2006

nonn

Primes arising in A122280 as greatest common divisor.

Cf. A122280.

{m=100;z=2*m;w=vectorsmall(z);b=1;a=2;w[b]=1;w[a]=1;stop=0;n=3;while(!stop&&n<=m,j=1;while(j<=z&&(w[j]==1||!isprime(p=gcd(a,j))),j++);if(j>z,stop=1,print1(p,",");a=j;w[a]=1;n++))}

A166133 After initial 1,2,4, a(n+1) is the smallest divisor of a(n)^2-1 that has not yet appeared in the sequence.

Original entry on oeis.org

1, 2, 4, 3, 8, 7, 6, 5, 12, 11, 10, 9, 16, 15, 14, 13, 21, 20, 19, 18, 17, 24, 23, 22, 69, 28, 27, 26, 25, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 201, 80, 79

Offset: 1

Franklin T. Adams-Watters, Oct 07 2009

The initial 1,2,4 provides the smallest example with this rule that is not simply the integers in order, nor (apparently) ends with all divisors of a(n)^2-1 already present.
Apparently the sequence is infinite and includes every positive integer.
Apr 05 2015: John Mason has computed the first ten million terms. See link to zipped file. - N. J. A. Sloane, Apr 06 2015
The sequence contains many runs of incrementing and decrementing values. In the 1200 steps following the 4, there are 136 increments, 706 decrements, and 358 larger steps. What is the limiting distribution for these steps? [Click the "listen" button to appreciate these runs. - N. J. A. Sloane, Apr 03 2015]
After 3, 198, 270, 570, 522, 600, 822, and 882, we have a(n+1) = a(n)^2-1. Does this happen infinitely often? Cf. A256406, A256407.
A256543 gives numbers m such that a(m+1) = a(m)-1 or a(m+1) = a(m)+1. - Reinhard Zumkeller, Apr 01 2015
If this is a permutation, then A255833 is the inverse permutation. - M. F. Hasler, Apr 01 2015
a(A256703(n)+1) = a(A256703(n))^2 - 1. - Reinhard Zumkeller, Apr 08 2015
For n > 3: a(n) = A027750(a(n-1)^2-1, A256751(n)). - Reinhard Zumkeller, Apr 09 2015

			After a(24) = 22, the divisors of 22^2-1 = 483 are 1, 3, 7, 21, 23, 69, 161, and 483; 1, 3, 7, 21, and 23 have already occurred, so a(25) = 69.

Franklin T. Adams-Watters and N. J. A. Sloane, Table of n, a(n) for n = 1..20000 (first 1203 terms from Franklin T. Adams-Watters)
Hans Havermann, Log plot of 450000+ terms [Produced by Mathematica's ListLogPlot command]
Hans Havermann, Over-sized point-joined (over 250000 terms) graph of the sequence [Heavily clipped, which explains the strange appearance. - _N. J. A. Sloane_, Apr 01 2015]
John Mason, Table of n, a(n) for n = 1..711888 [10 megabytes]
John Mason, Table of n, a(n) for n = 1..2000000 [32 megabytes]
John Mason, Ten million terms (zipped file)
John Mason, Java program to generate this sequence, used to generate 10M terms, and some other associated sequences; it requires splitting into single classes for use.
N. J. A. Sloane and others, "Blog" about A166133

Cf. A166134, A000027, A122280, A005563, A256406, A256407, A027750, A005563, A256557, A256559.
For records see A256403, A256404.
Smallest missing numbers: A256405, A256408, A256409.
Cf. A256541 (first differences), A256543.
Inverse (conjectured): A255833.
Cf. A256564 (smallest prime factors), A244080 (largest prime factors), A256578 (largest proper divisors), A256542 (number of divisors).
Upper envelope: the sequence of pairs (A256422(n),A256423(n)).
Cf. A256703.
Cf. A256751.

import Data.List (delete); import Data.List.Ordered (isect)
a166133 n = a166133_list !! (n-1)
a166133_list = 1 : 2 : 4 : f (3:[5..]) 4 where
   f zs x = y : f (delete y zs) y where
            y = head $ isect (a027750_row' (x ^ 2 - 1)) zs
-- Reinhard Zumkeller, Apr 01 2015

s = {1, 2, 4}; e = 4; Do[d = Divisors[e^2 - 1]; i = 1;
While[MemberQ[s, d[[i]]], i++]; e = d[[i]]; AppendTo[s, e], {19997}]; s (* Hans Havermann, Apr 03 2015 *)

al(n,m=4,u=6)={local(ds,db);
u=bitor(u,1<

A354225 Lexicographically earliest sequence of distinct positive integers such that a(1) = 1 and for any n > 1, n / gcd(n, a(n)) and a(n) / gcd(n, a(n)) are prime.

Original entry on oeis.org

1, 3, 2, 6, 7, 4, 5, 12, 15, 14, 13, 8, 11, 10, 9, 24, 19, 27, 17, 28, 33, 26, 29, 16, 35, 22, 18, 20, 23, 42, 37, 48, 21, 38, 25, 54, 31, 34, 51, 56, 43, 30, 41, 52, 63, 58, 53, 32, 77, 70, 39, 44, 47, 36, 65, 40, 69, 46, 61, 84, 59, 74, 45, 96, 55, 78, 71

Offset: 1

Rémy Sigrist, May 20 2022

nonn

This sequence is a self-inverse permutation of the positive integers that preserves the number of prime divisors (with or without multiplicity).

			The first terms are:
  n   a(n)  g=gcd(n, a(n))  n/g  a(n)/g
  --  ----  --------------  ---  ------
   1     1               1    1       1
   2     3               1    2       3
   3     2               1    3       2
   4     6               2    2       3
   5     7               1    5       7
   6     4               2    3       2
   7     5               1    7       5
   8    12               4    2       3
   9    15               3    3       5
  10    14               2    5       7
  11    13               1   11      13
  12     8               4    3       2
  13    11               1   13      11
  14    10               2    7       5

Michael De Vlieger, Table of n, a(n) for n = 1..10000
Michael De Vlieger, Annotated log-log plot of a(n), n = 1..2^14, showing records in red, local minima in blue, highlighting primes in green, fixed points in gold, and composite prime powers in magenta.
Index entries for sequences that are permutations of the natural numbers

Cf. A122280.

nn = 120; c[] = 0; a[1] = c[1] = 1; u = 2; Do[k = u; While[Nand[c[k] == 0, AllTrue[{i/#, k/#}, PrimeQ] &@ GCD[i, k]], k++]; Set[{a[i], c[k]}, {k, i}]; If[k == u, While[c[u] > 0, u++]], {i, 2, nn}]; Array[a, nn] (* _Michael De Vlieger, May 22 2022 *)

s=0; for (n=1, 67, for (v=1, oo, if (!bittest(s, v) && (n==1 || (isprime(n/g=gcd(n,v)) && isprime(v/g))), print1 (v", "); s+=2^v; break)))

a(prime(2*n)) = prime(2*n-1) (where prime(n) denotes the n-th prime number).

cp's OEIS Frontend

A122281 Inverse permutation to sequence A122280.

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A122462 a(n) = gcd(b(n-1),b(n)), where b(i) = A122280(i).

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A166133 After initial 1,2,4, a(n+1) is the smallest divisor of a(n)^2-1 that has not yet appeared in the sequence.

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A354225 Lexicographically earliest sequence of distinct positive integers such that a(1) = 1 and for any n > 1, n / gcd(n, a(n)) and a(n) / gcd(n, a(n)) are prime.

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