20421661 has 4 divisors (see below), whose sum is σ = 21992572. Its totient is φ = 18850752.

The previous prime is 20421647. The next prime is 20421691. The reversal of 20421661 is 16612402.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 16612402 = 2 ⋅8306201.

It can be written as a sum of positive squares in 2 ways, for example, as 1664100 + 18757561 = 1290^2 + 4331^2 .

It is a cyclic number.

It is not a de Polignac number, because 20421661 - 2⁵ = 20421629 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 (20421691) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 785436 + ... + 785461.

It is an arithmetic number, because the mean of its divisors is an integer number (5498143).

Almost surely, 2^20421661 is an apocalyptic number.

It is an amenable number.

20421661 is a deficient number, since it is larger than the sum of its proper divisors (1570911).

20421661 is a wasteful number, since it uses less digits than its factorization.

20421661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1570910.

The product of its (nonzero) digits is 576, while the sum is 22.

The square root of 20421661 is about 4519.0331930624. The cubic root of 20421661 is about 273.3361171968.

The spelling of 20421661 in words is "twenty million, four hundred twenty-one thousand, six hundred sixty-one".