A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1688544 can be evenly divided by 1 2 3 4 6 8 9 11 12 13 16 18 22 24 26 32 33 36 39 41 44 48 52 66 72 78 82 88 96 99 104 117 123 132 143 144 156 164 176 198 208 234 246 264 286 288 312 328 352 369 396 416 429 451 468 492 528 533 572 624 656 738 792 858 902 936 984 1,056 1,066 1,144 1,248 1,287 1,312 1,353 1,476 1,584 1,599 1,716 1,804 1,872 1,968 2,132 2,288 2,574 2,706 2,952 3,168 3,198 3,432 3,608 3,744 3,936 4,059 4,264 4,576 4,797 5,148 5,412 5,863 5,904 6,396 6,864 7,216 8,118 8,528 9,594 10,296 10,824 11,726 11,808 12,792 13,728 14,432 16,236 17,056 17,589 19,188 20,592 21,648 23,452 25,584 32,472 35,178 38,376 41,184 43,296 46,904 51,168 52,767 64,944 70,356 76,752 93,808 105,534 129,888 140,712 153,504 187,616 211,068 281,424 422,136 562,848 844,272 and 1,688,544, with no remainder.
Since 1,688,544 cannot be divided by just 1 and 1,688,544, it is not a prime number.