66628781 has 2 divisors, whose sum is σ = 66628782. Its totient is φ = 66628780.

The previous prime is 66628739. The next prime is 66628789. The reversal of 66628781 is 18782666.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 59521225 + 7107556 = 7715^2 + 2666^2 .

It is a cyclic number.

It is not a de Polignac number, because 66628781 - 2¹⁸ = 66366637 is a prime.

It is a super-3 number, since 3×66628781³ (a number of 24 digits) contains 333 as substring. Note that it is a super-d number also for d = 2.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (66628789) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 33314390 + 33314391.

It is an arithmetic number, because the mean of its divisors is an integer number (33314391).

Almost surely, 2^66628781 is an apocalyptic number.

It is an amenable number.

66628781 is a deficient number, since it is larger than the sum of its proper divisors (1).

66628781 is an equidigital number, since it uses as much as digits as its factorization.

66628781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 193536, while the sum is 44.

The square root of 66628781 is about 8162.6454657788. The cubic root of 66628781 is about 405.4033090579.

The spelling of 66628781 in words is "sixty-six million, six hundred twenty-eight thousand, seven hundred eighty-one".