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A173070 Palindromic mountain numbers. 2
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
Sequence in context: A134328 A113614 A134941 * A044867 A162531 A050697
KEYWORD
nonn,base,fini,full
AUTHOR
Omar E. Pol, Feb 09 2010
STATUS
approved

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Last modified February 25 00:32 EST 2024. Contains 370308 sequences. (Running on oeis4.)