20421 has 6 divisors (see below), whose sum is σ = 29510. Its totient is φ = 13608.

The previous prime is 20411. The next prime is 20431. The reversal of 20421 is 12402.

It is an interprime number because it is at equal distance from previous prime (20411) and next prime (20431).

It can be written as a sum of positive squares in only one way, i.e., 12321 + 8100 = 111^2 + 90^2 .

It is not a de Polignac number, because 20421 - 2⁵ = 20389 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (9), and also a Moran number because the ratio is a prime number: 2269 = 20421 / (2 + 0 + 4 + 2 + 1).

It is an Ulam number.

It is a Duffinian number.

It is a nialpdrome in base 12.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20411) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1126 + ... + 1143.

2²⁰⁴²¹ is an apocalyptic number.

It is an amenable number.

20421 is a deficient number, since it is larger than the sum of its proper divisors (9089).

20421 is a wasteful number, since it uses less digits than its factorization.

20421 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2275 (or 2272 counting only the distinct ones).

The product of its (nonzero) digits is 16, while the sum is 9.

The square root of 20421 is about 142.9020643658. The cubic root of 20421 is about 27.3333168088. Note that the first 4 decimals are identical.

Adding to 20421 its reverse (12402), we get a palindrome (32823).

It can be divided in two parts, 204 and 21, that added together give a square (225 = 15²).

The spelling of 20421 in words is "twenty thousand, four hundred twenty-one", and thus it is an iban number.