21009661 has 8 divisors (see below), whose sum is σ = 21749504. Its totient is φ = 20273760.

The previous prime is 21009643. The next prime is 21009679. The reversal of 21009661 is 16690012.

It is an interprime number because it is at equal distance from previous prime (21009643) and next prime (21009679).

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21009661 - 2⁵ = 21009629 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 (21009691) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 13426 + ... + 14908.

It is an arithmetic number, because the mean of its divisors is an integer number (2718688).

Almost surely, 2^21009661 is an apocalyptic number.

It is an amenable number.

21009661 is a deficient number, since it is larger than the sum of its proper divisors (739843).

21009661 is a wasteful number, since it uses less digits than its factorization.

21009661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1971.

The product of its (nonzero) digits is 648, while the sum is 25.

The square root of 21009661 is about 4583.6296752683. The cubic root of 21009661 is about 275.9347190352.

Adding to 21009661 its reverse (16690012), we get a palindrome (37699673).

The spelling of 21009661 in words is "twenty-one million, nine thousand, six hundred sixty-one".