666

Number
$666$ (six hundred and sixty-six) is:


 * $2 \times 3^2 \times 37$


 * The $36$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $325$, $351$, $378$, $406$, $435$, $465$, $496$, $528$, $561$, $595$, $630$:
 * $666 = \displaystyle \sum_{k \mathop = 1}^{36} k = \dfrac {36 \times \left({36 + 1}\right)} 2$


 * The $8$th palindromic triangular number after $1$, $3$, $6$, $55$, $66$, $171$, $595$.


 * The $3$rd and last after $55$, $66$ of the $3$ repdigit numbers which are also triangular.


 * The $7$th after $4$, $13$, $38$, $87$, $208$, $377$ in the sequence formed by adding the squares of the first $n$ primes:
 * $666 = \displaystyle \sum_{i \mathop = 1}^7 {p_i}^2 = 2^2 + 3^2 + 5^2 + 7^2 + 11^2 + 13^2 + 17^2$


 * The basis of an approximation to the Golden Ratio correct to $10$ decimal places:
 * $\phi \approx -2 \sin \left({666}\right) = -1.61803 \, 39887 \, 5 \ldots$


 * The Euler $\phi$ function of $666$ equals the product of its digits:
 * $\phi \left({666}\right) = 6 \times 6 \times 6$