1561216781 has 8 divisors (see below), whose sum is σ = 1621370784. Its totient is φ = 1502146800.

The previous prime is 1561216753. The next prime is 1561216801. The reversal of 1561216781 is 1876121651.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 1561216781 - 2⁶ = 1561216717 is a prime.

It is a super-2 number, since 2×1561216781² = 4874795674552003922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1561216381) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 268070 + ... + 273831.

It is an arithmetic number, because the mean of its divisors is an integer number (202671348).

Almost surely, 2^1561216781 is an apocalyptic number.

It is an amenable number.

1561216781 is a deficient number, since it is larger than the sum of its proper divisors (60154003).

1561216781 is an equidigital number, since it uses as much as digits as its factorization.

1561216781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 542011.

The product of its digits is 20160, while the sum is 38.

The square root of 1561216781 is about 39512.2358390411. The cubic root of 1561216781 is about 1160.0794587366.

The spelling of 1561216781 in words is "one billion, five hundred sixty-one million, two hundred sixteen thousand, seven hundred eighty-one".