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

101, 121, 131, 141, 151, 161, 171, 181, 191, 202, 212, 232, 242, 252, 262, 272, 282, 292, 303, 313, 323, 343, 353, 363, 373, 383, 393, 404, 414, 424, 434, 454, 464, 474, 484, 494, 505, 515, 525, 535, 545, 565, 575, 585, 595, 606, 616, 626, 636, 646, 656

Offset: 1

Author

Eric W. Weisstein

Keywords

• nonn
• base

References

• C. A. Pickover, ``The Undulation of the Monks.'' Ch. 20 in Keys to Infinity. New York: W.H.Freeman, pp. 159-161 1995.
• C. A. Pickover, "Wonders of Numbers", Oxford New York 2001, Chapter 52, pp. 123-124, 316-317.

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
• C. A. Pickover, "Wonders of Numbers, Adventures in Mathematics, Mind and Meaning," Zentralblatt review
• Eric Weisstein's World of Mathematics, Undulating Number

Crossrefs

Cf. A033619, A046076.

Cf. A252664.

Programs

Haskell

import Data.Set (fromList, deleteFindMin, insert)
a046075 n = a046075_list !! (n-1)
a046075_list = f $ fromList
               [100 * a + 10 * b + a | a <- [1..9], b <- [0..9], b /= a]
   where f s = m : f (insert (10 * m + div (mod m 100) 10) s')
               where (m, s') = deleteFindMin s
-- Reinhard Zumkeller, Apr 29 2015, May 01 2012