21001661 has 4 divisors (see below), whose sum is σ = 21016512. Its totient is φ = 20986812.

The previous prime is 21001657. The next prime is 21001667. The reversal of 21001661 is 16610012.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21001661 - 2² = 21001657 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21001667) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 5051 + ... + 8216.

It is an arithmetic number, because the mean of its divisors is an integer number (5254128).

Almost surely, 2^21001661 is an apocalyptic number.

It is an amenable number.

21001661 is a deficient number, since it is larger than the sum of its proper divisors (14851).

21001661 is a wasteful number, since it uses less digits than its factorization.

21001661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 14850.

The product of its (nonzero) digits is 72, while the sum is 17.

The square root of 21001661 is about 4582.7569213302. The cubic root of 21001661 is about 275.8996913718.

Adding to 21001661 its reverse (16610012), we get a palindrome (37611673).

The spelling of 21001661 in words is "twenty-one million, one thousand, six hundred sixty-one".