Are there any prime numbers with more than two digits such that all combinations of its digits (preserving order) are prime? For example, if the number abc is prime, then a, b, and c are prime, and so are ab, ac, and bc.
What is the largest such number, or the largest known? I'm not even sure this is possible, but I imagine the probability of a prime passing this test gets really small really fast as you start adding digits...
There is no such number. All digits except the first would have to be $3$, $7$ or $9$, since $1$ is not a prime and the remaining digits cannot be the last digits of primes with more than one digit. It cannot contain the same digit twice, since that would yield a number divisible by $11$. It also can't contain $39$ or $93$, which are divisible by $3$. That leaves only $37$, $73$, $97$ and $79$, preceded by one prime digit other than $3$ or $7$, i.e. $2$ or $5$. None of the eight resulting numbers is prime.