A357963 a(1)=1, a(2)=2. Thereafter, if there are prime divisors p of a(n-1) which do not divide a(n-2), a(n) is the least novel multiple of any such p. Otherwise a(n) is the least novel multiple of the squarefree kernel of a(n-1). See comments.

1, 2, 4, 6, 3, 9, 12, 8, 10, 5, 15, 18, 14, 7, 21, 24, 16, 20, 25, 30, 22, 11, 33, 27, 36, 26, 13, 39, 42, 28, 56, 70, 35, 105, 45, 60, 32, 34, 17, 51, 48, 38, 19, 57, 54, 40, 50, 80, 90, 63, 49, 77, 44, 46, 23, 69, 66, 52, 65, 55, 88, 58, 29, 87, 72, 62, 31, 93

Offset: 1

Author

David James Sycamore, Oct 22 2022

Keywords

• nonn

Comments

In other words, if a(n-1) has k prime divisors p_j, 1 <= j <= k which do not divide a(n-2), where 1 <= k <= omega(a(n-1)), and if m_j*p_j is the least multiple of p_j which is not already a term, then a(n) = Min{m_j*p_j; 1<=j<=k}. Otherwise every prime divisor of a(n-1) also divides a(n-2), in which case a(n) is the least multiple of the squarefree kernel of a(n-1) which is not already a term. If a(n-1) and a(n-2) are coprime the computation of a(n) ranges over all prime divisors of a(n-1). This happens only once (n=3), after which all adjacent terms share a common divisor (as in the EKG sequence, A064413).

Departs from A064413 and A352187 at a(19), a(31) respectively, and apparently shares the way odd primes are proven to appear in the former and conjectured to appear in the latter; namely as 2*p, p, 3*p.

Conjectures: Permutation of the positive integers with primes in natural order, appearing in same way as in EKG.

From Michael De Vlieger, Oct 22 2022 (Start)

An algorithm similar to the Rains algorithm for the EKG sequence efficiently generates the sequence.

Like the EKG sequence, this sequence forces primes into divisibility; Primes divide their predecessors and successors. Consequently they exhibit Lagarias-Rains-Sloane chain 2p -> p -> 3p outside of p = 2, just as in the EKG sequence.

Let us define several quasi-rays conspicuous in the scatterplot. From lowest to highest apparent slope, we have the following:

- beta: local minima, i.e., a(1)=1 and primes p in order.

- gamma: 2p, 4p, and certain other composites.

- alpha-k: k*p from large k to k = 3. This system appears as a series of fine quasi-rays, with 3p generally comprising records.

Records are 3p outside of {1, 2, 4, 12, 18, 24, 25, 30, 36, 42, 56, 70, 105}.

a(33) = 35 behaves like a prime; 70 precedes and 105 follows it. a(34) = 105 is conspicuous as it appears earlier than expected. (End)

Examples

			a(1)=1, a(2)=2 and 2 divides 2 but does not divide 1. Since 2 is the only prime divisor of 2, a(3) = 4, the least unused multiple of 2.
Since every prime divisor of a(3)=4 also divides a(2)=2, a(4) = 6, the least novel multiple of the squarefree kernel of 4.
a(19), a(20)=25, 30, and 30 has two prime divisors (2,3) which do not divide 25. The least multiples of 2, 3 not seen already are 22, 27 respectively, so a(21)=22.
a(29), a(30)=42, 28 and every prime dividing 28 (2,7) also divides 42, so a(31) is 56, the least novel multiple of 14 (squarefree kernel of 28).
		

Links

• Michael De Vlieger, Table of n, a(n) for n = 1..10000
• Michael De Vlieger, Scatterplot of a(n), n = 1..120, highlighting primes p in red, 2p in blue, and 3p in green.
• Michael De Vlieger, Log-log scatterplot of a(n), n = 1..2^12, labeling the first 64 terms, showing records in red and local minima in blue, highlighting primes in green and other prime powers in gold. As n increases, the quasi-ray 5*p shows prominently just under the records.
• Michael De Vlieger, Log-log scatterplot of a(n), n = 1..2^20, demonstrating fine quasi-rays.

Crossrefs

Cf. A001221, A064413, A352187, A336957, A357942, A357992, A357994, A358267.

Programs

Mathematica

Block[{a, c, f, g, k, m, q, nn}, nn = 68; c[] = False; q[] = 1; Array[Set[{a[#], c[#]}, {#, True}] &, 2]; q[2] = 2; Do[m = FactorInteger[a[n - 1]][[All, 1]]; f = Select[m, CoprimeQ[#, a[n - 2]] &]; If[Length[f] == 0, While[Set[k, # q[#]]; c[k], q[#]++] &[Times @@ m], Set[{k, q[#1]}, {#2, #2/#1}] & @@ First@ MinimalBy[Map[{#, Set[g, q[#]]; While[c[g #], g++]; # g} &, f], Last] ]; Set[{a[n], c[k]}, {k, True}], {n, 3, nn}]; Array[a, nn] ] (* Michael De Vlieger, Oct 22 2022 *)

Extensions

More terms from Michael De Vlieger, Oct 22 2022