Interleave numbers from 1 to n, with the same numbers reversed
JavaScript (ES6), 30 bytes

f=(n,k=1)=&gt;n?f(n-1,k+1)+n+k:''


How?

This is pretty straightforward but it's worth noting that the string is built from tail to head. An empty string at the beginning is appended last and allows the coercion of the final result to a string to happen.

Below is the detail of the recursion for f(4):

f(4) =                                            // initial call
f(4, 1) =                                         // applying the default value to k
f(3, 2) + 4 + 1 =                                 // recursive call #1
(f(2, 3) + 3 + 2) + 4 + 1 =                       // recursive call #2
((f(1, 4) + 2 + 3) + 3 + 2) + 4 + 1 =             // recursive call #3
(((f(0, 5) + 1 + 4) + 2 + 3) + 3 + 2) + 4 + 1 =   // recursive call #4
((('' + 1 + 4) + 2 + 3) + 3 + 2) + 4 + 1 =        // n = 0 --&gt; end of recursion
'' + 1 + 4 + 2 + 3 + 3 + 2 + 4 + 1 =              // final sum
'14233241'                                        // final result


Test cases



f=(n,k=1)=&gt;n?f(n-1,k+1)+n+k:''

console.log(f(1))
console.log(f(4))
console.log(f(26))
console.log(f(100))