phi(n)=phi(n+2)
\[p\]
is an odd prime.Then
\[\phi (2(2p-1))= \phi (2) \phi (2p-1)\]
Where
\[\phi (n)\]
is the number of integers up to \[n\]
which are prime relative to \[n\]
.If
\[2p-1\]
is also prime, then\[\phi (2p-1)=2p-2=2(p-1)= 2 \phi (p)= \phi (4p)= \phi (2(2p-1))\]
When
\[p, \; 2(2p-1)=n\]
are odd primes then \[\phi (n)= \phi (n+2)\]
. 
