A217866 - OEIS

%I #15 Oct 16 2012 14:26:41

%S 1,5,18,46,137,384,1059,2905,8066,21883,59743,162247,441824

%N Last time n appears in the first differences of n*log(n): A217865.

%C Conjecture: the last time n appears is always before the first time n+2 appears.

%C For example, examine: 1, 2, 2, 3, 2, 3, 3, 3, 4, 3, 3, 4, 3, 4, 4, 4, 4, 3, 4, 4, 5, 4, 4, 4 (A217865).

%C We can see that the last 2 is before the first 4, the last 3 is before the first 5, etc.

%e 2 lasts appears in A217865 at position 5 so a(2)=5.

%o (JavaScript)

%o function lastIndex(arr,k) {

%o p=-1;

%o for (a=1;a<arr.length;a++) if (arr[a]==k) p=a;

%o return p;

%o }

%o d=new Array();

%o for (i=1;i<500000;i++)

%o d[i]=Math.floor((i+1)*Math.log(i+1))-Math.floor(i*Math.log(i));

%o for (i=1;i<100;i++) document.write(lastIndex(d,i)+", ");

%Y Cf. A217865.

%Y Cf. A217976.

%K nonn

%O 1,2

%A _Jon Perry_, Oct 13 2012