21405872561 has 2 divisors, whose sum is σ = 21405872562. Its totient is φ = 21405872560.

The previous prime is 21405872539. The next prime is 21405872629. The reversal of 21405872561 is 16527850412.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19656040000 + 1749832561 = 140200^2 + 41831^2 .

It is a cyclic number.

It is not a de Polignac number, because 21405872561 - 2¹⁰ = 21405871537 is a prime.

It is a super-2 number, since 2×21405872561² (a number of 21 digits) contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21405872261) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10702936280 + 10702936281.

It is an arithmetic number, because the mean of its divisors is an integer number (10702936281).

Almost surely, 2^21405872561 is an apocalyptic number.

It is an amenable number.

21405872561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21405872561 is an equidigital number, since it uses as much as digits as its factorization.

21405872561 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 134400, while the sum is 41.

The spelling of 21405872561 in words is "twenty-one billion, four hundred five million, eight hundred seventy-two thousand, five hundred sixty-one".