21771561 has 16 divisors (see below), whose sum is σ = 33264000. Its totient is φ = 12407808.

The previous prime is 21771557. The next prime is 21771619. The reversal of 21771561 is 16517712.

It is not a de Polignac number, because 21771561 - 2² = 21771557 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21771521) 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, 8275 + ... + 10583.

It is an arithmetic number, because the mean of its divisors is an integer number (2079000).

Almost surely, 2^21771561 is an apocalyptic number.

21771561 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21771561 is a deficient number, since it is larger than the sum of its proper divisors (11492439).

21771561 is a wasteful number, since it uses less digits than its factorization.

21771561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2768.

The product of its digits is 2940, while the sum is 30.

The square root of 21771561 is about 4666.0005357908. Note that the first 3 decimals coincide. The cubic root of 21771561 is about 279.2307157514.

The spelling of 21771561 in words is "twenty-one million, seven hundred seventy-one thousand, five hundred sixty-one".