21661521 has 16 divisors (see below), whose sum is σ = 34147200. Its totient is φ = 11950848.

The previous prime is 21661517. The next prime is 21661531. The reversal of 21661521 is 12516612.

It is not a de Polignac number, because 21661521 - 2² = 21661517 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21661491 and 21661500.

It is not an unprimeable number, because it can be changed into a prime (21661531) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 17176 + ... + 18393.

It is an arithmetic number, because the mean of its divisors is an integer number (2134200).

Almost surely, 2^21661521 is an apocalyptic number.

21661521 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21661521 is a deficient number, since it is larger than the sum of its proper divisors (12485679).

21661521 is a wasteful number, since it uses less digits than its factorization.

21661521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 35608.

The product of its digits is 720, while the sum is 24.

The square root of 21661521 is about 4654.1939151694. The cubic root of 21661521 is about 278.7594823401.

The spelling of 21661521 in words is "twenty-one million, six hundred sixty-one thousand, five hundred twenty-one".