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A173070 Palindromic mountain numbers. 1
1, 121, 131, 141, 151, 161, 171, 181, 191, 12321, 12421, 12521, 12621, 12721, 12821, 12921, 13431, 13531, 13631, 13731, 13831, 13931, 14541, 14641, 14741, 14841, 14941, 15651, 15751, 15851, 15951, 16761, 16861, 16961, 17871, 17971, 18981 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

There are 256 terms, the last of which is 12345678987654321. - Michael S. Branicky, Aug 04 2022

LINKS

Michael S. Branicky, Table of n, a(n) for n = 1..256 (full sequence)

EXAMPLE

13731 is in the sequence because it is a palindrome (A002113) and it is also a mountain number (A134941).

. . . . .

. . . . .

. . 7 . .

. . . . .

. . . . .

. . . . .

. 3 . 3 .

. . . . .

1 . . . 1

PROG

(Python)

from itertools import chain, combinations as combs

def c(s): return s[0] == s[-1] == 1 and s == s[::-1]

ups = list(chain.from_iterable(combs(range(10), r) for r in range(2, 11)))

s = set(L[:-1] + R[::-1] for L in ups for R in ups if L[-1] == R[-1])

afull = [1] + sorted(int("".join(map(str, t))) for t in s if c(t))

print(afull[:40]) # Michael S. Branicky, Aug 04 2022

CROSSREFS

Cf. A002113, A134941, A134951, A134810, A134853, A173071

Sequence in context: A134328 A113614 A134941 * A044867 A162531 A050697

Adjacent sequences:  A173067 A173068 A173069 * A173071 A173072 A173073

KEYWORD

nonn,base,fini,full

AUTHOR

Omar E. Pol, Feb 09 2010

STATUS

approved

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Last modified October 6 21:32 EDT 2022. Contains 357270 sequences. (Running on oeis4.)