21991219781 has 2 divisors, whose sum is σ = 21991219782. Its totient is φ = 21991219780.

The previous prime is 21991219727. The next prime is 21991219807. The reversal of 21991219781 is 18791219912.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21734130625 + 257089156 = 147425^2 + 16034^2 .

It is a cyclic number.

It is not a de Polignac number, because 21991219781 - 2⁶ = 21991219717 is a prime.

It is a super-2 number, since 2×21991219781² (a number of 21 digits) contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21991219981) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10995609890 + 10995609891.

It is an arithmetic number, because the mean of its divisors is an integer number (10995609891).

Almost surely, 2^21991219781 is an apocalyptic number.

It is an amenable number.

21991219781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21991219781 is an equidigital number, since it uses as much as digits as its factorization.

21991219781 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 163296, while the sum is 50.

The spelling of 21991219781 in words is "twenty-one billion, nine hundred ninety-one million, two hundred nineteen thousand, seven hundred eighty-one".