1561971221 has 2 divisors, whose sum is σ = 1561971222. Its totient is φ = 1561971220.

The previous prime is 1561971181. The next prime is 1561971223. The reversal of 1561971221 is 1221791651.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1191216196 + 370755025 = 34514^2 + 19255^2 .

It is a cyclic number.

It is not a de Polignac number, because 1561971221 - 2⁶ = 1561971157 is a prime.

Together with 1561971223, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1561971223) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 780985610 + 780985611.

It is an arithmetic number, because the mean of its divisors is an integer number (780985611).

Almost surely, 2^1561971221 is an apocalyptic number.

It is an amenable number.

1561971221 is a deficient number, since it is larger than the sum of its proper divisors (1).

1561971221 is an equidigital number, since it uses as much as digits as its factorization.

1561971221 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 7560, while the sum is 35.

The square root of 1561971221 is about 39521.7816020483. The cubic root of 1561971221 is about 1160.2662936500.

The spelling of 1561971221 in words is "one billion, five hundred sixty-one million, nine hundred seventy-one thousand, two hundred twenty-one".