21581358187561 has 4 divisors (see below), whose sum is σ = 21738886349652. Its totient is φ = 21423830025472.

The previous prime is 21581358187549. The next prime is 21581358187573. The reversal of 21581358187561 is 16578185318512.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21581358187549) and next prime (21581358187573).

It can be written as a sum of positive squares in 2 ways, for example, as 4298755342336 + 17282602845225 = 2073344^2 + 4157235^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21581358187561 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21581358187561.

It is not an unprimeable number, because it can be changed into a prime (21581358157561) 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 in 3 ways as a sum of consecutive naturals, for example, 78764080840 + ... + 78764081113.

It is an arithmetic number, because the mean of its divisors is an integer number (5434721587413).

Almost surely, 2^{21581358187561} is an apocalyptic number.

It is an amenable number.

21581358187561 is a deficient number, since it is larger than the sum of its proper divisors (157528162091).

21581358187561 is a wasteful number, since it uses less digits than its factorization.

21581358187561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 157528162090.

The product of its digits is 16128000, while the sum is 61.

The spelling of 21581358187561 in words is "twenty-one trillion, five hundred eighty-one billion, three hundred fifty-eight million, one hundred eighty-seven thousand, five hundred sixty-one".