21591781 has 2 divisors, whose sum is σ = 21591782. Its totient is φ = 21591780.

The previous prime is 21591769. The next prime is 21591793. The reversal of 21591781 is 18719512.

It is a happy number.

It is a balanced prime because it is at equal distance from previous prime (21591769) and next prime (21591793).

It can be written as a sum of positive squares in only one way, i.e., 21538881 + 52900 = 4641^2 + 230^2 .

It is a cyclic number.

It is not a de Polignac number, because 21591781 - 2⁵ = 21591749 is a prime.

It is a super-2 number, since 2×21591781² = 932410013503922, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21591761) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10795890 + 10795891.

It is an arithmetic number, because the mean of its divisors is an integer number (10795891).

Almost surely, 2^21591781 is an apocalyptic number.

It is an amenable number.

21591781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21591781 is an equidigital number, since it uses as much as digits as its factorization.

21591781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 5040, while the sum is 34.

The square root of 21591781 is about 4646.6957077046. The cubic root of 21591781 is about 278.4600021850.

The spelling of 21591781 in words is "twenty-one million, five hundred ninety-one thousand, seven hundred eighty-one".