This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A381760 #21 Mar 15 2025 04:22:11
%S A381760 1,1,1,3,11,38,130,638,4158,23384,124520,847484,6987380,53746000,
%T A381760 400346544,3529108816,35963592624,351432650816,3346590201888,
%U A381760 36341624453568
%N A381760 Number of words of length 2n+1 with one 0 entry and two entries of each of 1..n so that there are exactly k numbers between two equal k's and so that the first element is not 0 and also does not exceed the last.
%H A381760 <a href="https://bbs.emath.ac.cn/thread-17154-1-1.html">Chinese BBS for the problem</a>
%F A381760 a(n) = A381759(n) - A176127(n).
%e A381760 For n=4, there are 3 different ways {131423024, 141302432, 240231413} so that a(4)=3 and both 023421314 and 041312432 are invalid since 0 in boundary.
%Y A381760 Cf. A014552, A176127, A381759.
%K A381760 nonn,more
%O A381760 1,4
%A A381760 _Zhao Hui Du_, Mar 06 2025