Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #5 Mar 31 2012 12:36:48
%S 1,1,2,4,9,23,65,201,679,2489,9831,41673,188858,911216,4664879,
%T 25265271,144350862,867748340,5475788662,36194252078,250093494424,
%U 1803204139718,13543620370665,105800880875545,858375849401380,7222921375625584
%N Number of n-element nonnegative integer arrays that are ascent sequences (interior element no greater than one plus up-steps preceding it) in both forward and reverse directions
%H R. H. Hardin, <a href="/A202552/b202552.txt">Table of n, a(n) for n = 1..116</a>
%e Some solutions for n=7
%e ..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0
%e ..0....1....1....0....1....0....0....1....1....1....0....1....0....0....1....1
%e ..1....1....0....1....0....0....0....2....2....0....1....0....1....1....0....1
%e ..2....1....0....0....1....0....1....1....1....0....1....2....0....2....0....1
%e ..1....0....1....1....2....0....0....0....0....1....1....2....0....2....2....1
%e ..1....0....0....1....1....0....1....1....0....1....0....1....0....1....1....1
%e ..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0
%K nonn
%O 1,3
%A _R. H. Hardin_ Dec 21 2011