1681

Number
$1681$ (one thousand six hundred and eighty-one) is:


 * $41^2$


 * The $6$th square whose decimal representation can be split into two parts which are each themselves square:
 * $1681 = 41^2$; $16 = 4^2$, $81 = 9^2$
 * and the only such number of $4$ digits whose two parts are both of $2$ digits


 * The $36$th positive integer which cannot be expressed as the sum of a square and a prime:
 * $1$, $10$, $25$, $34$, $58$, $64$, $85$, $\ldots$, $706$, $730$, $771$, $784$, $841$, $1024$, $1089$, $1225$, $1255$, $1351$, $1414$, $1444$, $1521$, $1681$, $\ldots$


 * The $41$st square number after $1$, $4$, $9$, $16$, $25$, $36$, $\ldots$, $1024$, $1089$, $1156$, $1225$, $1296$, $1369$, $1444$, $1521$, $1600$:
 * $1681 = 41 \times 41$