21851561 has 2 divisors, whose sum is σ = 21851562. Its totient is φ = 21851560.

The previous prime is 21851549. The next prime is 21851573. The reversal of 21851561 is 16515812.

It is a balanced prime because it is at equal distance from previous prime (21851549) and next prime (21851573).

It can be written as a sum of positive squares in only one way, i.e., 16000000 + 5851561 = 4000^2 + 2419^2 .

It is a cyclic number.

It is not a de Polignac number, because 21851561 - 2²² = 17657257 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21851521) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10925780 + 10925781.

It is an arithmetic number, because the mean of its divisors is an integer number (10925781).

Almost surely, 2^21851561 is an apocalyptic number.

It is an amenable number.

21851561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21851561 is an equidigital number, since it uses as much as digits as its factorization.

21851561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2400, while the sum is 29.

The square root of 21851561 is about 4674.5653273861. The cubic root of 21851561 is about 279.5723104883.

The spelling of 21851561 in words is "twenty-one million, eight hundred fifty-one thousand, five hundred sixty-one".