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OEIS A002113, palindromes in base 10, shows a number of Mathematica constructs. I have used:

pals = {0}; r = 2; Do[
 Do[AppendTo[pals, 
   n*10^(IntegerLength[n] - 1) + 
    FromDigits@Rest@Reverse@IntegerDigits[n]], {n, 10^(e - 1), 10^e - 1}];
 Do[AppendTo[pals, 
   n*10^IntegerLength[n] + FromDigits@Reverse@IntegerDigits[n]], {n, 
   10^(e - 1), 10^e - 1}], {e, r}]; pals

... with r = 7 but going up to r = 8 would take too long. Might there be a way to recode this so that r = 8 can be accomplished in a reasonable amount of time? I'm prepared to do weeks, but not months.

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    $\begingroup$ It is very likely that using ParallelDo on a computer with 4 or more processors/cores is going to put you in the "waiting for weeks" time-frame. $\endgroup$
    – Anton Antonov
    Commented Feb 17, 2020 at 18:33
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    $\begingroup$ What is r? Number of digits? $\endgroup$
    – MikeY
    Commented Feb 17, 2020 at 19:02
  • $\begingroup$ Using a specific integer r will generate a list of all palindromes less than 10^(2r). So, for r=2, up to 9999; for r=3, up to 999999. $\endgroup$
    – Hans Havermann
    Commented Feb 17, 2020 at 19:47

2 Answers 2

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palindromeNdigits[n_Integer?Positive]:=
  Tuples[Array[Boole[#==1]~Range~9&,⌈n/2⌉,Boole[n!=1]]][[;;,Array[Min[#,n+1-#]&,n]]
  ].(10^Range[0,n-1]);

palindromeGen[max_Integer?NonNegative]:=
  Join@@Array[palindromeNdigits,Max[Ceiling@Log10@max,1]]//Pick[#,UnitStep[max-#],1]&;

palindromeNdigits[2]

{11, 22, 33, 44, 55, 66, 77, 88, 99}

palindromeGen[500]

{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 22, 33, 44, 55, 66, 77, 88, 99,
101, 111, 121, 131, 141, 151, 161, 171, 181, 191, 202, 212, 222, 232,
242, 252, 262, 272, 282, 292, 303, 313, 323, 333, 343, 353, 363, 373,
383, 393, 404, 414, 424, 434, 444, 454, 464, 474, 484, 494}

palindromeGen[10^10] // Total // AbsoluteTiming

{0.0257969, 545045045045040}

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  • $\begingroup$ I ran palindromeGen[10^18] on a recently acquired 2020 iMac i9 and it created the two billion (less one) terms in 493 seconds! $\endgroup$
    – Hans Havermann
    Commented Apr 28, 2022 at 15:40
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Every positive integer with two digits $\{a,b\}$ generates the palindromes $\{a,b,b,a\}$ and $\{a,b,a\}$. This generalizes to more than two digits.

PalindromeGen[max_] :=
   Block[{d, n, lim},
      lim = If[max < 11, 10, Ceiling[10^(1/2 + Ceiling[Log[10, max]]/2)]];
      Sort@Pick[d = Flatten[
        Table[
           d = IntegerDigits[n];
           {FromDigits[Join[d, Reverse[d]]], 
            FromDigits[Join[d, Rest[Reverse[d]]]]},
        {n, 1, lim}]],
        UnitStep[max - d], 1]]

For example,

AbsoluteTiming[Total[PalindromeGen[10^10]]]

{1.29566, 545045045045040}

An upper limit of $10^{12}$ took about 13s, corresponding to your $r=6$. An upper limit of $10^{14}$ took 133s.

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