21279781 has 2 divisors, whose sum is σ = 21279782. Its totient is φ = 21279780.

The previous prime is 21279757. The next prime is 21279787. The reversal of 21279781 is 18797212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21077281 + 202500 = 4591^2 + 450^2 .

It is a cyclic number.

It is not a de Polignac number, because 21279781 - 2⁵ = 21279749 is a prime.

It is a super-2 number, since 2×21279781² = 905658158815922, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21279781.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21279787) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10639890 + 10639891.

It is an arithmetic number, because the mean of its divisors is an integer number (10639891).

Almost surely, 2^21279781 is an apocalyptic number.

It is an amenable number.

21279781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21279781 is an equidigital number, since it uses as much as digits as its factorization.

21279781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 14112, while the sum is 37.

The square root of 21279781 is about 4613.0013006718. The cubic root of 21279781 is about 277.1122458206.

It can be divided in two parts, 212797 and 81, that added together give a triangular number (212878 = T₆₅₂).

The spelling of 21279781 in words is "twenty-one million, two hundred seventy-nine thousand, seven hundred eighty-one".