6,661,661,161

Number
$6 \, 661 \, 661 \, 161$ is:
 * $81 \, 619^2$


 * The $24$th (and largest known) square after $1$, $4$, $9$, $16$, $25$, $\ldots$, $11 \, 881$, $29 \, 929$, $44 \, 944$, $55 \, 225$, $69 \, 696$, $9 \, 696 \, 996$ which has no more than $2$ distinct digits


 * The $81 \, 619$th square number:
 * $6 \, 661 \, 661 \, 161 = 81 \, 619 \times 81 \, 619$