1521

Number
$1521$ (one thousand five hundred and twenty-one) is:


 * $3^2 \times 13^2$


 * The $39$th square number after $1$, $4$, $9$, $16$, $25$, $36$, $\ldots$, $625$, $676$, $729$, $784$, $841$, $900$, $961$, $1024$, $1089$, $1156$, $1225$, $1296$, $1369$, $1444$:
 * $1521 = 39 \times 39$


 * The $35$th positive integer which cannot be expressed as the sum of a square and a prime:
 * $1$, $10$, $25$, $34$, $58$, $64$, $85$, $\ldots$, $706$, $730$, $771$, $784$, $841$, $1024$, $1089$, $1225$, $1255$, $1351$, $1414$, $1444$, $1521$, $\ldots$