%I A089973
%S A089973 1,6,24,48,25440,554400
%N A089973 Number of maximum-length 2-surprising sequences in n symbols.
%C A089973 A sequence of symbols is 2-surprising if, for every pair of symbols X 
               and Y, not necessarily distinct and every distance D, there is at 
               most one position in t he sequence where X precedes Y by distance 
               D. The maximum lengths are given in A0899 72.
%D A089973 Dennis E. Shasha, Puzzling Adventures, Scientific American 289(#12, 2003), 
               22.
%e A089973 The 2-surprising sequences in 3 symbols are:
%e A089973 (2012100) (1021200) (1120210) (2011210) (1202210) (2210120) (2101120) 
               (1022120)
%e A089973 (2100201) (0021201) (0212201) (2102011) (0120211) (2010021) (2201021) 
               (0122021)
%e A089973 (1200102) (0121102) (0012102) (1020012) (0211012) (1102012) (1201022) 
               (0210122)
%e A089973 Thus a(3)=24.
%Y A089973 Cf. A008062.
%Y A089973 Sequence in context: A000056 A083170 A087081 this_sequence A033581 A009943 
               A028595
%Y A089973 Adjacent sequences: A089970 A089971 A089972 this_sequence A089974 A089975 
               A089976
%K A089973 nonn
%O A089973 1,2
%A A089973 John W. Layman (layman(AT)math.vt.edu), Nov 17 2003