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A046075 Nontrivial undulants; base 10 numbers >100 which are of the form aba, abab, ababa, ..., where a != b.

%I A046075 #22 Feb 16 2025 08:32:38
%S A046075 101,121,131,141,151,161,171,181,191,202,212,232,242,252,262,272,282,
%T A046075 292,303,313,323,343,353,363,373,383,393,404,414,424,434,454,464,474,
%U A046075 484,494,505,515,525,535,545,565,575,585,595,606,616,626,636,646,656
%N A046075 Nontrivial undulants; base 10 numbers >100 which are of the form aba, abab, ababa, ..., where a != b.
%D A046075 C. A. Pickover, ``The Undulation of the Monks.'' Ch. 20 in Keys to Infinity. New York: W.H.Freeman, pp. 159-161 1995.
%D A046075 C. A. Pickover, "Wonders of Numbers", Oxford New York 2001, Chapter 52, pp. 123-124, 316-317.
%H A046075 Reinhard Zumkeller, <a href="/A046075/b046075.txt">Table of n, a(n) for n = 1..10000</a>
%H A046075 C. A. Pickover, "Wonders of Numbers, Adventures in Mathematics, Mind and Meaning," <a href="http://www.zentralblatt-math.org/zmath/en/search/?q=an:0983.00008&amp;format=complete">Zentralblatt review</a>
%H A046075 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/UndulatingNumber.html">Undulating Number</a>
%o A046075 (Haskell)
%o A046075 import Data.Set (fromList, deleteFindMin, insert)
%o A046075 a046075 n = a046075_list !! (n-1)
%o A046075 a046075_list = f $ fromList
%o A046075                [100 * a + 10 * b + a | a <- [1..9], b <- [0..9], b /= a]
%o A046075    where f s = m : f (insert (10 * m + div (mod m 100) 10) s')
%o A046075                where (m, s') = deleteFindMin s
%o A046075 -- _Reinhard Zumkeller_, Apr 29 2015, May 01 2012
%Y A046075 Cf. A033619, A046076.
%Y A046075 Cf. A252664.
%K A046075 nonn,base
%O A046075 1,1
%A A046075 _Eric W. Weisstein_