It is a semiprime because it is the product of two primes.
It is a cyclic number.
It is a de Polignac number, because none of the positive numbers 2k-20721247 is a prime.
It is a congruent number.
Almost surely, 220721247 is an apocalyptic number.
20721247 is an equidigital number, since it uses as much as digits as its factorization.
20721247 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 232912.
The square root of 20721247 is about 4552.0596437217. The cubic root of 20721247 is about 274.6662485527. Note that the first 3 decimals are identical.
Adding to 20721247 its reverse (74212702), we get a palindrome (94933949).
The spelling of 20721247 in words is "twenty million, seven hundred twenty-one thousand, two hundred forty-seven".