21731561 has 2 divisors, whose sum is σ = 21731562.
Its totient is φ = 21731560.
The previous prime is 21731539. The next prime is 21731579. The reversal of 21731561 is 16513712.
It is a strong prime.
It can be written as a sum of positive squares in only one way, i.e., 20976400 + 755161 = 4580^2 + 869^2
It is a cyclic number.
It is not a de Polignac number, because 21731561 - 210 = 21730537 is a prime.
It is a Chen prime.
It is a self number, because there is not a number n which added to its sum of digits gives 21731561.
It is not a weakly prime, because it can be changed into another prime (21731161) by changing a digit.
It is a pernicious number, because its binary representation contains a prime number (13) of ones.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10865780 + 10865781.
It is an arithmetic number, because the mean of its divisors is an integer number (10865781).
Almost surely, 221731561 is an apocalyptic number.
It is an amenable number.
21731561 is a deficient number, since it is larger than the sum of its proper divisors (1).
21731561 is an equidigital number, since it uses as much as digits as its factorization.
21731561 is an odious number, because the sum of its binary digits is odd.
The product of its digits is 1260, while the sum is 26.
The square root of 21731561 is about 4661.7122390813.
The cubic root of 21731561 is about 279.0596045194.
The spelling of 21731561 in words is "twenty-one million, seven hundred thirty-one thousand, five hundred sixty-one".