Does disabling 'The Nether' improve server performance?

www.telegraph.co.uk and telegraph.co.uk are two totally different addresses. Place 127.0.0.1 www.telegraph.co.uk in your hosts file as well.

This is because the www. is simply a subdomain of telegraph.co.uk, not a particularly special part of the web. They simply both lead to the same place - but the hosts file is not very clever, you have to be very clear and verbose when blocking things.

Jelly, 26 23 bytes

Forward

Ѷp9¶7ÆR2ĿV€$ÆPÐf$ÐĿFị@

Try it online!

Words

Ñ ¶ p 9 ¶ 7ÆR2ĿV€$ÆPÐf$ÐĿFị@

Backward

7ÆR2ĿV€$ÆPÐf$ÐĿFị@¶9p¶Ñ

Try it online!

Words

7ÆR2ĿV€$ÆPÐf$ÐĿFị@ ¶ 9 p ¶ Ñ

How it works

All Jelly programs consist of links (Jelly's take on functions), which are separated by linefeeds or pilcrows (¶). The last of them is the main link; it is called automatically when the program is run.

The forward program works as follows.

Ñ                   Helper link. Unused.


p9                  Helper link. Take the Cartesian product with [1, ..., 9].


7ÆR2ĿV€$ÆPÐf$ÐĿFị@  Main link. Argument: n

7ÆR                 Yield all primes up to 7.
             ÐĿ     
            $ÐĿ     Combine the two quicklinks to the left into a monadic chain,
                    and call it repeatedly until the results are no longer unique.
                    Return the array of all intermediate results.
       $              Combine the two links to the left into a monadic chain.
   2Ŀ               Call the helper link on line 2.
     Ṿ€                 Eval each array in the product. This casts to string
                        before evaluating, thus concatenating both numbers.
        ÆPÐf        Filter by primality; keep only primes.
               F    Flatten the resulting array.
                ị@  Retrieve the element at index n.

The backward program does almost exactly the same; there are only two differences.

• The main link is now Ñ, which simply calls the link below it (wrapping around), i.e., the main link of the forward program.

• 9p instead of p9 return the reversed Cartesian product.

$H$ is not a subgroup of $G$. For example, $H$ contains $1/2$ but does not contain $(1/2)^{-1}=2$. So naturally constructions that require $H$ to be a subgroup can't be expected to work.