portrait: 36

36 is both the square of six, and the eighth triangular number or the sum of the first eight non-zero positive integers, which makes 36 the first non-trivial square triangular number. Aside from being the smallest square triangular number other than 1, it is also the only triangular number (other than 1) whose square root is also a triangular number. 36 is also the eighth refactorable number, as it has exactly nine positive divisors, and 9 is one of them; in fact, it is the smallest positive integer with at least nine divisors, which leads 36 to be the 7th highly composite number.It is the sum of the fourth pair of twin-primes (17 + 19),and the 18th Harshad number in decimal, as it is divisible by the sum of its digits (9).