20421551 has 4 divisors (see below), whose sum is σ = 20443032. Its totient is φ = 20400072.

The previous prime is 20421503. The next prime is 20421553. The reversal of 20421551 is 15512402.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 20421551 - 2⁶ = 20421487 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20421553) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 9245 + ... + 11238.

It is an arithmetic number, because the mean of its divisors is an integer number (5110758).

Almost surely, 2^20421551 is an apocalyptic number.

20421551 is a deficient number, since it is larger than the sum of its proper divisors (21481).

20421551 is an equidigital number, since it uses as much as digits as its factorization.

20421551 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 21480.

The product of its (nonzero) digits is 400, while the sum is 20.

The square root of 20421551 is about 4519.0210223012. The cubic root of 20421551 is about 273.3356264267.

Adding to 20421551 its reverse (15512402), we get a palindrome (35933953).

The spelling of 20421551 in words is "twenty million, four hundred twenty-one thousand, five hundred fifty-one".