69

Number
$69$ (sixty-nine) is:
 * $3 \times 23$


 * The only integer whose square and cube use each of the digits from $0$ to $9$ exactly once each:
 * $69^2 = 4761$, $69^3 = 328 \, 509$


 * The $4$th after $21$, $29$, $61$ of the $5$ $2$-digit positive integers which can occur as a $5$-fold repetition at the end of a square number


 * The $20$th Ulam number after $1$, $2$, $3$, $4$, $6$, $8$, $11$, $13$, $16$, $18$, $26$, $28$, $36$, $38$, $47$, $48$, $53$, $57$, $62$:
 * $69 = 16 + 53$


 * The $24$th semiprime:
 * $69 = 3 \times 23$


 * The $27$th of $35$ integers less than $91$ to which $91$ itself is a Fermat pseudoprime:
 * $3$, $4$, $9$, $10$, $12$, $16$, $17$, $22$, $23$, $25$, $27$, $29$, $30$, $36$, $38$, $40$, $43$, $48$, $51$, $53$, $55$, $61$, $62$, $64$, $66$, $68$, $69$, $\ldots$


 * The $30$th odd positive integer that cannot be expressed as the sum of exactly $4$ distinct non-zero square numbers all of which are coprime
 * $1$, $\ldots$, $37$, $41$, $43$, $45$, $47$, $49$, $53$, $55$, $59$, $61$, $67$, $69$, $\ldots$

Also see

 * Number whose Square and Cube use all Digits Once