21651781 has 2 divisors, whose sum is σ = 21651782. Its totient is φ = 21651780.

The previous prime is 21651769. The next prime is 21651793. The reversal of 21651781 is 18715612.

It is a balanced prime because it is at equal distance from previous prime (21651769) and next prime (21651793).

It can be written as a sum of positive squares in only one way, i.e., 18533025 + 3118756 = 4305^2 + 1766^2 .

It is a cyclic number.

It is not a de Polignac number, because 21651781 - 2¹⁹ = 21127493 is a prime.

It is a super-2 number, since 2×21651781² = 937599240943922, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21651701) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10825890 + 10825891.

It is an arithmetic number, because the mean of its divisors is an integer number (10825891).

Almost surely, 2^21651781 is an apocalyptic number.

It is an amenable number.

21651781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21651781 is an equidigital number, since it uses as much as digits as its factorization.

21651781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3360, while the sum is 31.

The square root of 21651781 is about 4653.1474294288. The cubic root of 21651781 is about 278.7176951174.

The spelling of 21651781 in words is "twenty-one million, six hundred fifty-one thousand, seven hundred eighty-one".