# [><>], 276 bytes 1$1-:?!v$: 1[0$ >:?!va2[$:{:@%:{$-{,]v >$n; v < ^ >~{]02.1+1+ffr+1r< :}[r]{ [01>:{*@@+$a*l2=?!^~]+ff+9+1g"3"=?v"3"ff+9+1pf0. :}[l01-$> $:0(?v$@$:@@:@)?v@@1-$}v >"2"ff+9+1p00. >. ^-1l v!?} < .4a}$@@$< .4a< ^26{]r0[}:{]~{< [Try it online!][TIO-jj1p1zei] Call this one with `-v n` to get the n-th element (1-indexed) 1$1-:?!;$::n84*o1[0$ >:?!va2[$:{:@%:{$-{,]v v < ^ >~{]02.1+1+ffr+1r< :}[r]{ [01>:{*@@+$a*l2=?!^~]+ff+9+1g"3"=?v"3"ff+9+1pf0. :}[l01-$> $:0(?v$@$:@@:@)?v@@1-$}v >"2"ff+9+1p00. >. ^-1l v!?} < .4a}$@@$< .4a< ^26{]r0[}:{]~{< [Try it online!][TIO-jj1n69s9] Call with `-v n` to get a list of n-1 elements starting at 1 [Online Fish Interpreter][1] A long and complex one, this is [OEIS A004000][2]. > Let a(n) = k, form m by Reversing the digits of k, Add m to k Then > Sort the digits of the sum into increasing order to get a(n+1). > > Example: 668 -> 668 + 866 = 1534 -> 1345. Because the code in ><> is quite long, the challenge is to crack it for \$n = 34\$. These 34 elements make a total of 290 bytes, just a bit too much to hardcode the result ;) Here are the 34 first elements 1-indexed 1 2 4 8 16 77 145 668 1345 6677 13444 55778 133345 666677 1333444 5567777 12333445 66666677 133333444 556667777 1233334444 5566667777 12333334444 55666667777 123333334444 556666667777 1233333334444 5566666667777 12333333334444 55666666667777 123333333334444 556666666667777 1233333333334444 5566666666667777 [><>]: https://esolangs.org/wiki/Fish [TIO-jj1n69s9]: https://tio.run/##VY7BSsUwEEX3/YppGUEbU5K8Ijr2JfMfJYVuqkJRqZBNSH89pj4VvYvhDncOc5eXj@ecNWpJrn5Eotf7vn3To0L4lS1RmM2IFImvKKKMtz5U8EcB/mkAmA5wj16ZTgstlmUTehsqoDRuPgKMSluKLbPAuV3N2dXT7suZeBD6qTk1ZxfKvOzvi@q@yFVpifb4gKSuXUBGYia@cYG5ROnSwzbmh1SFtN13rUnqFULt0lGw6@eEzFjs4YdqMnfRb2pMFP0eh5yzDHDqPwE "><> – Try It Online" [TIO-jj1p1zei]: https://tio.run/##VY7BasQwDETv@QolqNCu62B7l0LVrK3/CA7kkm0hlJKCL8b5ddfe7RY6BzHiadAsH9/vOWvUklwbkOAqPSqEP9mKZjMiReIHiijjsw/NneLnGwT4pwFgqmiPXpleCy2WZRN6GxqgNG4@AoxKW4oHZoHzYTVn1067L2fiVehLd@zOLpR5278W1V@Tq9ISbf2ApB5dQEZiJn5ygbmgdOthO3NPqpK0/W@tSeoVQutSLdif5oTMWGz1QzOZl@g3NSaKfo9DzlkGOJ5@AA "><> – Try It Online" [1]: https://fishlanguage.com/ [2]: http://oeis.org/A004000