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