51

This number is a composite.

The smallest semiprime that is the product of two primes whose concatenation (in some order) produce the decimal expansion of square root of 3, e.g., 51 = 17 *3, 346 = 173 * 2. The sequence begins 51, 346, 8660254037844386465, 9135637898642163233468659, 91356386175449607536090017841537606328361073633, .... . See the largest known. [Bajpai]