21661651 has 4 divisors (see below), whose sum is σ = 23630904. Its totient is φ = 19692400.

The previous prime is 21661643. The next prime is 21661657. The reversal of 21661651 is 15616612.

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

It is a cyclic number.

It is not a de Polignac number, because 21661651 - 2³ = 21661643 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21661657) 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, 984610 + ... + 984631.

It is an arithmetic number, because the mean of its divisors is an integer number (5907726).

Almost surely, 2^21661651 is an apocalyptic number.

21661651 is a deficient number, since it is larger than the sum of its proper divisors (1969253).

21661651 is a wasteful number, since it uses less digits than its factorization.

21661651 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1969252.

The product of its digits is 2160, while the sum is 28.

The square root of 21661651 is about 4654.2078810470. The cubic root of 21661651 is about 278.7600399904.

The spelling of 21661651 in words is "twenty-one million, six hundred sixty-one thousand, six hundred fifty-one".