21212761 has 4 divisors (see below), whose sum is σ = 21223332. Its totient is φ = 21202192.

The previous prime is 21212749. The next prime is 21212777. The reversal of 21212761 is 16721212.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 5760000 + 15452761 = 2400^2 + 3931^2 .

It is a cyclic number.

It is not a de Polignac number, because 21212761 - 2¹³ = 21204569 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21212701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1246 + ... + 6631.

It is an arithmetic number, because the mean of its divisors is an integer number (5305833).

Almost surely, 2^21212761 is an apocalyptic number.

It is an amenable number.

21212761 is a deficient number, since it is larger than the sum of its proper divisors (10571).

21212761 is an equidigital number, since it uses as much as digits as its factorization.

21212761 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10570.

The product of its digits is 336, while the sum is 22.

The square root of 21212761 is about 4605.7313208653. The cubic root of 21212761 is about 276.8210211087.

Adding to 21212761 its reverse (16721212), we get a palindrome (37933973).

The spelling of 21212761 in words is "twenty-one million, two hundred twelve thousand, seven hundred sixty-one".