cp's OEIS Frontend

A372975 a(1) = 1, a(2) = 2; for n > 2, a(n) is the smallest unused positive number such that a(n) shares a factor with a(n-1) while omega(a(n)) does not equal omega(a(n-1)).

%I A372975 #33 May 29 2024 07:06:07
%S A372975 1,2,6,3,12,4,10,5,15,9,18,8,14,7,21,27,24,16,20,25,30,22,11,33,42,26,
%T A372975 13,39,60,28,32,34,17,51,66,36,64,38,19,57,78,40,70,35,49,56,84,44,90,
%U A372975 45,81,48,102,46,23,69,105,50,110,52,114,54,120,55,121,77,126,58,29,87,132,62,31,93,138
%N A372975 a(1) = 1, a(2) = 2; for n > 2, a(n) is the smallest unused positive number such that a(n) shares a factor with a(n-1) while omega(a(n)) does not equal omega(a(n-1)).
%C A372975 The sequence shows similar behavior to the EKG sequence A064413; for the terms studied the primes appear in the natural order, and when a prime p is a term, the proceeding and following terms are 2p and 3p respectively.
%C A372975 For larger n a graph of the sequence also displays very similar behavior to A064413, although for the first ~2500 terms the main concentration of terms are along two lines which eventually join - see the attached image of the first 5000 terms.
%C A372975 The fixed points begin 1, 2, 22, 26, 36, 38, 1991, 2023, 2159, 2189, 2627; it is likely no more exist. The sequence is conjectured to be a permutation of the positive integers.
%C A372975 From _Michael De Vlieger_, May 28 2024: (Start)
%C A372975 Four general trajectories become apparent in log log scatterplot:
%C A372975 1. Beta, the trajectory of primes a(j) = p.
%C A372975 2. Alpha, the trajectory of numbers a(j+1) = 3*p.
%C A372975 3. Delta, the trajectory of perfect prime powers and numbers k with omega(k) = 3.
%C A372975 4. Gamma, the trajectory of all other (composite) numbers.
%C A372975 Delta begins with a(21) = 30 and merges with gamma around n = 2958. The merger alters the "slope" of all trajectories as a result. Thereafter, a number k with omega(k) = 2 is comparable in size with one that has omega(k) = 3. This does not seem to happen for omega(k) = 4, etc. (See a(2102) = 2310).
%C A372975 Perfect prime powers may technically constitute a separate, more scattered trajectory superposed upon delta. Still, the merger with gamma seems to occur around the same point as with delta.
%C A372975 Exception to first comment: 12 follows 3, since omega(9) = omega(3). The number 12 lies outside trajectory alpha, since 12 = 4*3. (End)
%H A372975 Scott R. Shannon, <a href="/A372975/b372975.txt">Table of n, a(n) for n = 1..10000</a>
%H A372975 Scott R. Shannon, <a href="/A372975/a372975.png">Image of the first 5000 terms</a>. The green line is a(n) = n.
%H A372975 Michael De Vlieger, <a href="/A372975/a372975_1.png">Log log scatterplot of a(n)</a>, n = 1..2^16, showing primes in red, perfect prime powers in gold, squarefree composites in green, and numbers neither squarefree nor prime powers in blue and purple, where purple represents powerful numbers that are not prime powers.
%e A372975 a(3) = 6 as a(2) = 2 and omega(2) = A001221(2) = 1, and 6 shares a factor with 2 while omega(6) = A001221(6) = 2 which does not equal 1.
%t A372975 nn = 1000; c[_] := False; m[_] := 1;
%t A372975 Array[Set[{a[#], c[#]}, {#, True}] &, 2]; j = 2; v = 4;
%t A372975 Do[Which[
%t A372975   And[PrimeQ[#], OddQ[#]] &[j/2], k = j/2,
%t A372975   PrimePowerQ[j], k = FactorInteger[j][[1, 1]];
%t A372975     While[Or[c[#], PrimePowerQ[#]] &[k*m[k]], m[k]++]; k *= m[k],
%t A372975   True, k = v;
%t A372975     While[Or[CoprimeQ[j, k], PrimeNu[k] == #, c[k]] &[PrimeNu[j]], k++]];
%t A372975   Set[{a[n], c[k], j}, {k, True, k}];
%t A372975   If[k == v, While[Or[PrimeQ[v], c[v]], v++]], {n, 3, nn}];
%t A372975 Array[a, nn] (* _Michael De Vlieger_, May 28 2024 *)
%Y A372975 Cf. A001221, A027748, A372974, A064413, A352588, A109465, A353916, A372974.
%K A372975 nonn
%O A372975 1,2
%A A372975 _Scott R. Shannon_, May 26 2024