Concatenating n with n + 1
Jelly, 3 bytes

ŻVƝ


A monadic link accepting an integer which yields a list of integers

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How?

ŻVƝ - Link: integer       e.g. 59
Ż   - zero-range               [0,1,2,3,4,5,6, ... ,58,59]
  Ɲ - apply to each pair: i.e: [0,1] or [5,6]  or  [58,59]
 V  -   evaluate* jelly code   1     or 56     or  5859
    -                       -&gt; [1,12,23,45,56, ... 5859]

* When given a list V actually joins the Python string values and evaluates that
  ...so e.g.: [58,59] -&gt; ['58','59'] -&gt; '5859' -&gt; 5859