21209661 has 16 divisors (see below), whose sum is σ = 34278720. Its totient is φ = 12854160.

The previous prime is 21209653. The next prime is 21209663. The reversal of 21209661 is 16690212.

It is not a de Polignac number, because 21209661 - 2³ = 21209653 is a prime.

It is a Smith number, since the sum of its digits (27) coincides with the sum of the digits of its prime factors.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21209663) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 35410 + ... + 36003.

It is an arithmetic number, because the mean of its divisors is an integer number (2142420).

Almost surely, 2^21209661 is an apocalyptic number.

It is an amenable number.

21209661 is a deficient number, since it is larger than the sum of its proper divisors (13069059).

21209661 is a wasteful number, since it uses less digits than its factorization.

21209661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 71433 (or 71427 counting only the distinct ones).

The product of its (nonzero) digits is 1296, while the sum is 27.

The square root of 21209661 is about 4605.3947713524. The cubic root of 21209661 is about 276.8075357201.

Adding to 21209661 its reverse (16690212), we get a palindrome (37899873).

It can be divided in two parts, 2120 and 9661, that added together give a triangular number (11781 = T₁₅₃).

The spelling of 21209661 in words is "twenty-one million, two hundred nine thousand, six hundred sixty-one".