21561712778 has 8 divisors (see below), whose sum is σ = 33216692772. Its totient is φ = 10489481856.

The previous prime is 21561712771. The next prime is 21561712789. The reversal of 21561712778 is 87721716512.

It can be written as a sum of positive squares in 2 ways, for example, as 17324614129 + 4237098649 = 131623^2 + 65093^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21561712771) 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, 145687175 + ... + 145687322.

Almost surely, 2^21561712778 is an apocalyptic number.

21561712778 is a deficient number, since it is larger than the sum of its proper divisors (11654979994).

21561712778 is a wasteful number, since it uses less digits than its factorization.

21561712778 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 291374536.

The product of its digits is 329280, while the sum is 47.

The spelling of 21561712778 in words is "twenty-one billion, five hundred sixty-one million, seven hundred twelve thousand, seven hundred seventy-eight".