81561421 has 2 divisors, whose sum is σ = 81561422. Its totient is φ = 81561420.

The previous prime is 81561419. The next prime is 81561457. The reversal of 81561421 is 12416518.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 80820100 + 741321 = 8990^2 + 861^2 .

It is a cyclic number.

It is not a de Polignac number, because 81561421 - 2¹ = 81561419 is a prime.

Together with 81561419, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 81561421.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81561521) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40780710 + 40780711.

It is an arithmetic number, because the mean of its divisors is an integer number (40780711).

Almost surely, 2^81561421 is an apocalyptic number.

It is an amenable number.

81561421 is a deficient number, since it is larger than the sum of its proper divisors (1).

81561421 is an equidigital number, since it uses as much as digits as its factorization.

81561421 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1920, while the sum is 28.

The square root of 81561421 is about 9031.1361965148. The cubic root of 81561421 is about 433.6722113822.

Adding to 81561421 its reverse (12416518), we get a palindrome (93977939).

The spelling of 81561421 in words is "eighty-one million, five hundred sixty-one thousand, four hundred twenty-one".