• 49 can be written using four 4's:
It is a happy number.
The square root of 49 is 7.
49 is nontrivially palindromic in base 6.
49 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.
49 is an esthetic number in base 6, base 9, base 11 and base 15, because in such bases its adjacent digits differ by 1.
It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 94 = 2 ⋅47.
49 is a Gilda number.
It is a magnanimous number.
It is an alternating number because its digits alternate between even and odd.
It is a Duffinian number.
49 is an undulating number in base 6.
49 is a lucky number.
It is a plaindrome in base 5, base 10, base 11, base 13, base 14 and base 15.
It is a nialpdrome in base 7, base 8, base 9, base 12 and base 16.
49 is the 7-th square number.
49 is the 4-th centered octagonal number.
It is an amenable number.
49 is an equidigital number, since it uses as much as digits as its factorization.
49 is an odious number, because the sum of its binary digits is odd.
The cubic root of 49 is about 3.6593057100.
Subtracting from 49 its sum of digits (13), we obtain a triangular number (36 = T8).
Multiplying 49 by its product of digits (36), we get a square (1764 = 422).
Subtracting 49 from its reverse (94), we obtain a triangular number (45 = T9).