OFFSET
1,1
COMMENTS
From Michael S. Branicky, Jun 13 2021: (Start)
All terms start and end with the digit 6, except those consisting of d+1 1's, which arise from palindromes of d 2's.
If a(n) > 6 starts with a 6, then its second and second-to-last digits are either both 0 or both 5; and if it has 5 or more digits, its third and third-to-last digits are either both 1 or both 6; thus, terms in the latter case only start with 111, 601, 606, 651, 656.
Using ^ to denote repeated concatenation, contains terms of the forms 1^(d+1), arising from palindromes of the form 2^d; (60)^d 6, arising from (12)^d 1; and (65)^d 6, arising from (13)^d 1; among other patterns.
(Conjectures)
All terms contain only the digits {0, 1, 5, 6}.
For d odd, the only term with d digits is 1^d; equivalently, all terms starting with 6 have odd length. (End)
LINKS
Michael S. Branicky, Table of n, a(n) for n = 1..8217 (all terms where p has <= 26 digits)
MATHEMATICA
Select[5Select[Range@100000, PalindromeQ]+1, PalindromeQ] (* Giorgos Kalogeropoulos, Jun 11 2021 *)
PROG
(Python)
from itertools import product
def ispal(n): s = str(n); return s == s[::-1]
def pals(d, base=10): # all positive d-digit palindromes
digits = "".join(str(i) for i in range(base))
for p in product(digits, repeat=d//2):
if d > 1 and p[0] == "0": continue
left = "".join(p); right = left[::-1]
for mid in [[""], digits][d%2]:
t = int(left + mid + right)
if t > 0: yield t
def ok(pal): return ispal(5*pal+1)
print([5*p+1 for d in range(1, 10) for p in pals(d) if ok(p)]) # Michael S. Branicky, Jun 11 2021
(PARI) is(n) = my(x=(n-1)/5, dn=digits(n), dx); if(x!=ceil(x), return(0)); dx=digits(x); dn==Vecrev(dn) && dx==Vecrev(dx) && n>1 \\ Felix Fröhlich, Jun 11 2021
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Amarnath Murthy and Meenakshi Srikanth (menakan_s(AT)yahoo.com), May 09 2003
EXTENSIONS
Corrected and extended by Ray Chandler, May 21 2003
STATUS
approved