21051661 has 4 divisors (see below), whose sum is σ = 22290012. Its totient is φ = 19813312.

The previous prime is 21051643. The next prime is 21051673. The reversal of 21051661 is 16615012.

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

It can be written as a sum of positive squares in 2 ways, for example, as 12460900 + 8590761 = 3530^2 + 2931^2 .

It is a cyclic number.

It is not a de Polignac number, because 21051661 - 2¹⁷ = 20920589 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 (21051691) 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, 619150 + ... + 619183.

It is an arithmetic number, because the mean of its divisors is an integer number (5572503).

Almost surely, 2^21051661 is an apocalyptic number.

It is an amenable number.

21051661 is a deficient number, since it is larger than the sum of its proper divisors (1238351).

21051661 is a wasteful number, since it uses less digits than its factorization.

21051661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1238350.

The product of its (nonzero) digits is 360, while the sum is 22.

The square root of 21051661 is about 4588.2089098035. The cubic root of 21051661 is about 276.1184685357.

Adding to 21051661 its reverse (16615012), we get a palindrome (37666673).

It can be divided in two parts, 21051 and 661, that added together give a palindrome (21712).

The spelling of 21051661 in words is "twenty-one million, fifty-one thousand, six hundred sixty-one".