21561209491 has 2 divisors, whose sum is σ = 21561209492. Its totient is φ = 21561209490.

The previous prime is 21561209479. The next prime is 21561209503. The reversal of 21561209491 is 19490216512.

It is a balanced prime because it is at equal distance from previous prime (21561209479) and next prime (21561209503).

It is a cyclic number.

It is not a de Polignac number, because 21561209491 - 2⁵ = 21561209459 is a prime.

It is not a weakly prime, because it can be changed into another prime (21561209431) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10780604745 + 10780604746.

It is an arithmetic number, because the mean of its divisors is an integer number (10780604746).

Almost surely, 2^21561209491 is an apocalyptic number.

21561209491 is a deficient number, since it is larger than the sum of its proper divisors (1).

21561209491 is an equidigital number, since it uses as much as digits as its factorization.

21561209491 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 38880, while the sum is 40.

The spelling of 21561209491 in words is "twenty-one billion, five hundred sixty-one million, two hundred nine thousand, four hundred ninety-one".