RTA (Reverse-Then-Add) root of a number
Perl 6 Raku, 45 44 39 bytes

{+(1...{$_∈($^a,{$_+.flip}...*&gt;$_)})}

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Explanation:
{                                   }  # Anonymous code block
 +(1...                            )     # Find the first integer
       {                          }      # Where
        $_∈                              # The input is an element of
           (              ...    )         # A sequence defined by
            $^b,                           # The first element is the number we're checking
                {$_+.flip}                 # Each element is the previous element plus its reverse
                             *&gt;$_          # The last element is larger than the input