20728421 has 12 divisors (see below), whose sum is σ = 24340140. Its totient is φ = 17599680.

The previous prime is 20728403. The next prime is 20728427. The reversal of 20728421 is 12482702.

It can be written as a sum of positive squares in 2 ways, for example, as 13764100 + 6964321 = 3710^2 + 2639^2 .

It is not a de Polignac number, because 20728421 - 2¹⁰ = 20727397 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 (20728427) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 3401 + ... + 7281.

It is an arithmetic number, because the mean of its divisors is an integer number (2028345).

Almost surely, 2^20728421 is an apocalyptic number.

It is an amenable number.

20728421 is a deficient number, since it is larger than the sum of its proper divisors (3611719).

20728421 is a wasteful number, since it uses less digits than its factorization.

20728421 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4004 (or 3997 counting only the distinct ones).

The product of its (nonzero) digits is 1792, while the sum is 26.

The square root of 20728421 is about 4552.8475704772. The cubic root of 20728421 is about 274.6979427249.

The spelling of 20728421 in words is "twenty million, seven hundred twenty-eight thousand, four hundred twenty-one".