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 A173070 #6 Aug 04 2022 11:00:46
%S A173070 1,121,131,141,151,161,171,181,191,12321,12421,12521,12621,12721,
%T A173070 12821,12921,13431,13531,13631,13731,13831,13931,14541,14641,14741,
%U A173070 14841,14941,15651,15751,15851,15951,16761,16861,16961,17871,17971,18981
%N A173070 Palindromic mountain numbers.
%C A173070 There are 256 terms, the last of which is 12345678987654321. - _Michael S. Branicky_, Aug 04 2022
%H A173070 Michael S. Branicky, <a href="/A173070/b173070.txt">Table of n, a(n) for n = 1..256</a> (full sequence)
%e A173070 13731 is in the sequence because it is a palindrome (A002113) and it is also a mountain number (A134941).
%e A173070 . . . . .
%e A173070 . . . . .
%e A173070 . . 7 . .
%e A173070 . . . . .
%e A173070 . . . . .
%e A173070 . . . . .
%e A173070 . 3 . 3 .
%e A173070 . . . . .
%e A173070 1 . . . 1
%o A173070 (Python)
%o A173070 from itertools import chain, combinations as combs
%o A173070 def c(s): return s[0] == s[-1] == 1 and s == s[::-1]
%o A173070 ups = list(chain.from_iterable(combs(range(10), r) for r in range(2, 11)))
%o A173070 s = set(L[:-1] + R[::-1] for L in ups for R in ups if L[-1] == R[-1])
%o A173070 afull = [1] + sorted(int("".join(map(str, t))) for t in s if c(t))
%o A173070 print(afull[:40]) # _Michael S. Branicky_, Aug 04 2022
%Y A173070 Cf. A002113, A134941, A134951, A134810, A134853, A173071
%K A173070 nonn,base,fini,full
%O A173070 1,2
%A A173070 _Omar E. Pol_, Feb 09 2010