81209501 has 2 divisors, whose sum is σ = 81209502. Its totient is φ = 81209500.

The previous prime is 81209489. The next prime is 81209503. The reversal of 81209501 is 10590218.

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., 51768025 + 29441476 = 7195^2 + 5426^2 .

It is a cyclic number.

It is not a de Polignac number, because 81209501 - 2⁶ = 81209437 is a prime.

Together with 81209503, 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 (81209503) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40604750 + 40604751.

It is an arithmetic number, because the mean of its divisors is an integer number (40604751).

Almost surely, 2^81209501 is an apocalyptic number.

It is an amenable number.

81209501 is a deficient number, since it is larger than the sum of its proper divisors (1).

81209501 is an equidigital number, since it uses as much as digits as its factorization.

81209501 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 720, while the sum is 26.

The square root of 81209501 is about 9011.6314283264. The cubic root of 81209501 is about 433.0475780079.

Adding to 81209501 its reverse (10590218), we get a palindrome (91799719).

The spelling of 81209501 in words is "eighty-one million, two hundred nine thousand, five hundred one".