172991981 has 2 divisors, whose sum is σ = 172991982. Its totient is φ = 172991980.

The previous prime is 172991963. The next prime is 172991999. The reversal of 172991981 is 189199271.

It is a balanced prime because it is at equal distance from previous prime (172991963) and next prime (172991999).

It can be written as a sum of positive squares in only one way, i.e., 167469481 + 5522500 = 12941^2 + 2350^2 .

It is an emirp because it is prime and its reverse (189199271) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 172991981 - 2²⁶ = 105883117 is a prime.

It is a super-2 number, since 2×172991981² = 59852450980608722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (172991921) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a good prime.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 86495990 + 86495991.

It is an arithmetic number, because the mean of its divisors is an integer number (86495991).

Almost surely, 2^172991981 is an apocalyptic number.

It is an amenable number.

172991981 is a deficient number, since it is larger than the sum of its proper divisors (1).

172991981 is an equidigital number, since it uses as much as digits as its factorization.

172991981 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 81648, while the sum is 47.

The square root of 172991981 is about 13152.6415977932. The cubic root of 172991981 is about 557.1968561137.

The spelling of 172991981 in words is "one hundred seventy-two million, nine hundred ninety-one thousand, nine hundred eighty-one".