55

Number
$55$ (fifty-five) is:
 * $5 \times 11$


 * The $10$th triangular number after $1, 3, 6, 10, 15, 21, 28, 36, 45$:
 * $55 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = \dfrac {10 \times \left({10 + 1}\right)} 2$


 * The $5$th heptagonal number after $1, 7, 18, 34$:
 * $55 = 1 + 7 + 11 + 16 + 21 = \dfrac {5 \left({5 \times 5 - 3}\right)} 2$


 * The $5$th square pyramidal number after $1, 5, 14, 30$:
 * $55 = 1 + 4 + 9 + 14 + 25 = \dfrac {5 \left({5 + 1}\right) \left({2 \times 5 + 1}\right)} 6$


 * The $19$th semiprime after $4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51$:
 * $55 = 5 \times 11$


 * The $10$th Fibonacci number, after $1, 1, 2, 3, 5, 8, 13, 21, 34$:
 * $55 = 21 + 34$


 * The $4$th Kaprekar number after $1, 9, 45$:
 * $55^2 = 3025 \to 30 + 25 = 55$


 * The $5$th and last after $0, 1, 3, 21$ of the $5$ Fibonacci numbers which are also triangular.


 * The $2$nd after $1$ of the $4$ square pyramidal numbers which are also triangular.


 * The $1$st of the $3$ repdigit numbers which are also triangular.


 * The $4$th palindromic triangular number after $1, 3, 6$.


 * The $1$st of the $4$ cubic recurring digital invariants:
 * $55 \to 250 \to 133 \to 55$


 * The $24$th and last after $1, 2, 4, 5, 6, 8, 9, 12, 13, 15, 16, 17, 20, 24, 25, 27, 28, 32, 35, 26, 39, 48, 51$ of the $24$ positive integers which cannot be expressed as the sum of distinct non-pythagorean primes.


 * There are exactly $55$ sets of $4$ integers $\left\{ {a, b, c, d}\right\}$ such that all integers can be written in the form:
 * $n = a x^2 + b y^2 + c z^2 + d w^2$
 * for integer $x, y, z, w$.

Also see

 * Sets of $4$ Integers $a, b, c, d$ for which Every Integer is in form $a x^2 + b y^2 + c z^2 + d u^2$