21256016124661 has 2 divisors, whose sum is σ = 21256016124662. Its totient is φ = 21256016124660.

The previous prime is 21256016124559. The next prime is 21256016124697. The reversal of 21256016124661 is 16642161065212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17570265056100 + 3685751068561 = 4191690^2 + 1919831^2 .

It is a cyclic number.

It is not a de Polignac number, because 21256016124661 - 2¹¹ = 21256016122613 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21256016194661) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (23) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10628008062330 + 10628008062331.

It is an arithmetic number, because the mean of its divisors is an integer number (10628008062331).

Almost surely, 2^{21256016124661} is an apocalyptic number.

It is an amenable number.

21256016124661 is a deficient number, since it is larger than the sum of its proper divisors (1).

21256016124661 is an equidigital number, since it uses as much as digits as its factorization.

21256016124661 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 207360, while the sum is 43.

Adding to 21256016124661 its reverse (16642161065212), we get a palindrome (37898177189873).

The spelling of 21256016124661 in words is "twenty-one trillion, two hundred fifty-six billion, sixteen million, one hundred twenty-four thousand, six hundred sixty-one".