login

Reminder: The OEIS is hiring a new managing editor, and the application deadline is January 26.

a(1) = 1; for n > 1, a(n) is the smallest positive number not occurring earlier that shares a factor with a(n-1) plus the number of divisors of a(n-1).
3

%I #13 Jan 16 2023 14:56:50

%S 1,2,4,7,3,5,14,6,8,9,10,12,15,19,18,16,21,20,13,24,22,26,25,28,17,38,

%T 27,31,11,39,43,30,32,34,36,33,37,42,35,45,48,29,62,40,44,46,50,49,52,

%U 54,56,58,60,51,55,59,61,57,122,63,23,65,66,64,71,73,69,146,68,70,72,74,75,78,76,41

%N a(1) = 1; for n > 1, a(n) is the smallest positive number not occurring earlier that shares a factor with a(n-1) plus the number of divisors of a(n-1).

%C The sequence is conjectured to be a permutation of the positive integers. In the first 250000 terms there are twenty-three fixed points: 1, 2, 12, 16, 27 ..., 2279, 5401, 7339. It is possibly no more exist although this is unknown.

%H Robert Israel, <a href="/A356432/b356432.txt">Table of n, a(n) for n = 1..10000</a>

%H Scott R. Shannon, <a href="/A356432/a356432.png">Image of the first 250000 terms</a>. The green line is y = n.

%e a(7) = 14 as a(6) = 5 which has two divisors, and 14 is the smallest unused number that shares a factor with 5 + 2 = 7.

%p A[1]:= 1; S:= {$2..5000}:

%p for i from 2 do

%p found:= false;

%p t:= A[i-1] + numtheory:-tau(A[i-1]);

%p for s in S do

%p if igcd(s,t) > 1 then

%p A[i]:= s;

%p found:= true;

%p S:= S minus {s};

%p break

%p fi

%p od;

%p if not found then break fi;

%p od:

%p seq(A[j],j=1..i-1); # _Robert Israel_, Jan 16 2023

%Y Cf. A356430, A356431, A000005, A354960, A348086.

%K nonn

%O 1,2

%A _Scott R. Shannon_, Aug 07 2022