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:
 * $1$, $10$, $25$, $34$, $58$, $64$, $85$, $\ldots$, $706$, $730$, $771$, $784$, $841$, $1024$, $1089$, $1225$, $1255$, $1351$, $1414$, $1444$, $1521$, $\ldots$