This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A193408 #39 Jul 29 2025 14:29:19
%S A193408 1,2,3,4,5,6,7,8,9,11,22,33,44,55,66,77,88,99,111,121,131,141,151,161,
%T A193408 171,181,191,222,232,242,252,262,272,282,292,333,343,353,363,373,383,
%U A193408 393,444,454,464,474,484,494,555,565,575,585,595,666,676,686,696,777,787,797,888,898,999,1111,1221,1231,1241,1251,1261,1271,1281,1291,1321,1331
%N A193408 Hill numbers.
%C A193408 Another version of mountain numbers (A134941) and A193407.
%C A193408 For n > 20 the structure of digits represents a hill. The first digit is equal to the last digit (1 - 9). The first digits are in nondecreasing order. The last digits are in nonincreasing order. The numbers may have more than one largest digit. Sequence is infinite.
%C A193408 Superset of mountain numbers (A134941), A193407, and Giza numbers (A134810).
%C A193408 Superset of A110784. - _R. J. Mathar_, Aug 07 2011
%e A193408 Illustration using a term of this sequence, 4566664:
%e A193408 . . 6 6 6 6 .
%e A193408 . 5 . . . . .
%e A193408 4 . . . . . 4
%t A193408 nonz[v_] := Select[v,#!=0 &]; hillQ[n_] := Module[{d=IntegerDigits[n]}, If[d[[1]] != d[[-1]], Return[False]]; MemberQ[{{},{0},{-2}}, nonz@ Differences@ Sign@ nonz@ Differences@d]]; Select[Range[1000], hillQ] (* _Amiram Eldar_, Dec 19 2018 *)
%Y A193408 Cf. A110784, A134810, A134941, A193407.
%K A193408 nonn,base
%O A193408 1,2
%A A193408 _Jaroslav Krizek_, Jul 25 2011