A081939 a(1) = 2; a(n+1) is the smallest palindrome > a(n) that has a common factor with a(n).

2, 4, 6, 8, 22, 33, 44, 55, 66, 77, 88, 99, 111, 141, 171, 222, 232, 242, 252, 262, 272, 282, 292, 404, 414, 424, 434, 444, 454, 464, 474, 484, 494, 585, 595, 616, 626, 636, 646, 656, 666, 676, 686, 696, 717, 747, 777, 828, 838, 848, 858, 868, 878, 888, 898

Offset

1,1

Comments

Palindromes with an even number of digits are divisible by 11, so when a(n)=A002113(k) and A055642(a(n)) and A055642(A002113(k+1)) are even, a(n+1)=A002113(k+1). - Robert Israel, Jul 04 2018

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Index entries for sequences related to palindromes

Maple

dmax:= 5: # to get all terms with at most dmax digits
revdigs:= proc(n)
  local L, Ln, i;
  L:= convert(n, base, 10);
  Ln:= nops(L);
  add(L[i]*10^(Ln-i), i=1..Ln);
end proc:
P:= $0..9:
for d from 2 to dmax do
  if d::even then
    P:= P, seq(10^(d/2)*x + revdigs(x), x=10^(d/2-1)..10^(d/2)-1)
  else
    m:= (d-1)/2;
    P:= P, seq(seq(10^(m+1)*x + 10^m*j+revdigs(x), j=0..9), x=10^(m-1)..10^m-1);
  fi
od:
P:= [P]:
r:= P[3]: Res:= r: count:= 1:
for i from 4 to nops(P) do
  if igcd(P[i], r) > 1 then
    count:= count+1; r:= P[i]; Res:= Res, r;
  fi
od:
Res; # Robert Israel, Jul 04 2018

Prog

(PARI) ispal(n) = my(d=digits(n)); d == Vecrev(d);
lista(nn) = {print1(last = 2, ", "); for (n=3, nn, if (ispal(n) && (gcd(n, last) != 1), print1(n, ", "); last = n; ); ); } \\ Michel Marcus, Aug 12 2015

Crossrefs

Cf. A002113, A055642, A083136, A081938.

Sequence in context: A045927 A165931 A321600 * A082615 A277258 A029951

Adjacent sequences: A081936 A081937 A081938 * A081940 A081941 A081942

Keyword

base,nonn

Author

Amarnath Murthy, Apr 02 2003

Extensions

More terms from David Wasserman, Jun 29 2004

Status

approved