49

This number is a composite.

The smallest composite number whose digits are composite such that the concatenated factors of either digit with the remaining digit(s) is prime. [Rivera]

The first 49 digits after the decimal point in e = 2.718... form a prime. [Honaker]

The largest prime square which is greater than the product of all lesser primes. [Murthy]

The smallest semiprime whose sum of divisors is also a semiprime. [Gupta]

phi(49)*49^49 + 1 is prime. The reversal of 49 has the same property. [Firoozbakht]

The smallest square that is the sum of three consecutive primes. [Silva]

The smallest prime square that is the sum of consecutive composite numbers in three ways, i.e., 24+25 = 15+16+18 = 4+6+8+9+10+12 = 7^2. [Loungrides]

100^49 + 49 is the largest non-titanic prime of form 100^n + n. [Loungrides]

49 is the smallest April Fools prime whose sum of digits is not divisible by π(-4+9). [Worrom]