21997661 has 8 divisors (see below), whose sum is σ = 25273344. Its totient is φ = 18755280.

The previous prime is 21997627. The next prime is 21997667. The reversal of 21997661 is 16679912.

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 21997661 - 2¹⁸ = 21735517 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 (21997667) 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, 6890 + ... + 9563.

It is an arithmetic number, because the mean of its divisors is an integer number (3159168).

Almost surely, 2^21997661 is an apocalyptic number.

It is an amenable number.

21997661 is a deficient number, since it is larger than the sum of its proper divisors (3275683).

21997661 is a wasteful number, since it uses less digits than its factorization.

21997661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 16651.

The product of its digits is 40824, while the sum is 41.

The square root of 21997661 is about 4690.1664149580. The cubic root of 21997661 is about 280.1940024560.

The spelling of 21997661 in words is "twenty-one million, nine hundred ninety-seven thousand, six hundred sixty-one".