cp's OEIS Frontend

A383421 a(1)=1; thereafter if a(n-1) is a first occurrence, then a(n) is the number of values that occur exactly once in the sequence thus far. Otherwise; a(n) is the number of terms that are the same distance away from an earlier equal value as a(n-1) is from its previous last occurrence.

%I A383421 #13 May 09 2025 22:33:49
%S A383421 1,1,1,2,1,2,3,1,2,3,4,1,2,3,4,5,1,3,6,2,5,4,6,7,1,2,2,3,2,4,3,6,5,2,
%T A383421 5,5,4,9,2,6,5,7,3,4,10,2,11,3,8,4,4,5,5,6,9,10,6,8,6,6,7,7,8,10,7,10,
%U A383421 7,8,12,2,3,5,10,13,3,9,8,8,9,11,5,9,12,12
%N A383421 a(1)=1; thereafter if a(n-1) is a first occurrence, then a(n) is the number of values that occur exactly once in the sequence thus far. Otherwise; a(n) is the number of terms that are the same distance away from an earlier equal value as a(n-1) is from its previous last occurrence.
%H A383421 Neal Gersh Tolunsky, <a href="/A383421/b383421.txt">Table of n, a(n) for n = 1..10000</a>
%H A383421 Sean A. Irvine, <a href="https://github.com/archmageirvine/joeis/blob/master/src/irvine/oeis/a383/A383421.java">Java program</a> (github)
%H A383421 Neal Gersh Tolunsky, <a href="/A383421/a383421.png">Graph of the first 400000 terms</a>
%e A383421 a(18) = 3: a(17) = 1 has a distance of 5 from its previous last occurrence a(12) = 1. There are a total of 3 terms with a distance of 5 from an earlier equal term; they occur at indices i = 8,9,17. So a(18) = 3.
%Y A383421 Cf. A383339, A383340.
%K A383421 nonn
%O A383421 1,4
%A A383421 _Neal Gersh Tolunsky_, Apr 26 2025