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Previous terms can appear in any order but can be used at most once. [From Paolo P. Lava, Mar 04 2010]
Table of n, a(n) for n=0..61.
137 is not in the sequence because 13 U 7 -> 137
797 is not in the sequence because 7 U 97 or 79 U 7 -> 797
Sequence in context: A092570 A133956 A176164 * A238647 A171821 A004051
Adjacent sequences: A141406 A141407 A141408 * A141410 A141411 A141412
Paolo P. Lava and Giorgio Balzarotti, Aug 04 2008, Aug 28 2008, Mar 04 2010