85

Number
$85$ (eighty-five) is:


 * $5 \times 17$


 * The $2$nd Fermat pseudoprime to base $4$ after $15$:
 * $4^{85} \equiv 4 \pmod {85}$


 * The $3$rd positive integer after $50$, $65$ which can be expressed as the sum of two square numbers in two or more different ways:
 * $85 = 9^2 + 2^2 = 7^2 + 6^2$


 * The $4$th non-square positive integer which cannot be expressed as the sum of a square and a prime:
 * $10$, $34$, $58$, $85$, $\ldots$


 * The $5$th Smith number after $4$, $22$, $27$, $58$:
 * $8 + 5 = 5 + 1 + 7 = 13$


 * The $7$th positive integer which cannot be expressed as the sum of a square and a prime:
 * $1$, $10$, $25$, $34$, $58$, $64$, $85$, $\ldots$


 * The $10$th positive integer $n$ after $0$, $1$, $5$, $25$, $29$, $41$, $49$, $61$, $65$ such that the Fibonacci number $F_n$ ends in $n$


 * The $16$th positive integer $n$ after $5$, $11$, $17$, $23$, $29$, $30$, $36$, $42$, $48$, $54$, $60$, $61$, $67$, $73$, $79$ such that no factorial of an integer can end with $n$ zeroes


 * The $27$th semiprime:
 * $85 = 5 \times 17$