Well, just another idea came up into my mind and i have no idea how to solve it 😀
Is there infinitely many prime numbers, which are not repunits and their inverse is also prime? (For example, inverse of 31 is 13 which is also prime. i didn't have any other name to describe the function!)
P.S:I now know they are called Emirps.
Number Theory – Is a Prime with Reversed Digits Still Prime?
elementary-number-theoryprime numbers
Best Answer
Actually, it has a name and it's quaintly called an "emirp". (The word "prime" in reverse.) The link given is to the Online Encyclopaedia of Integers' list,
$13, 17, 31, 37, 71, 73, 79, 97, 107, 113, 149, 157, 167,\dots$
While there are an infinite number of primes, I believe it is an open problem if there is an infinite number of emirps.
P.S. Regarding terminology, given $x$, then its "multiplicative inverse" (or "reciprocal") is $1/x$. For functions, for ex, given $\sin(x)$, then its "inverse function" is $\arcsin(x)$.