A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1317888 can be evenly divided by 1 2 3 4 6 8 9 11 12 13 16 18 22 24 26 32 33 36 39 44 48 52 64 66 72 78 88 96 99 104 117 128 132 143 144 156 176 192 198 208 234 256 264 286 288 312 352 384 396 416 429 468 512 528 572 576 624 704 768 792 832 858 936 1,024 1,056 1,144 1,152 1,248 1,287 1,408 1,536 1,584 1,664 1,716 1,872 2,112 2,288 2,304 2,496 2,574 2,816 3,072 3,168 3,328 3,432 3,744 4,224 4,576 4,608 4,992 5,148 5,632 6,336 6,656 6,864 7,488 8,448 9,152 9,216 9,984 10,296 11,264 12,672 13,312 13,728 14,976 16,896 18,304 19,968 20,592 25,344 27,456 29,952 33,792 36,608 39,936 41,184 50,688 54,912 59,904 73,216 82,368 101,376 109,824 119,808 146,432 164,736 219,648 329,472 439,296 658,944 and 1,317,888, with no remainder.
Since 1,317,888 cannot be divided by just 1 and 1,317,888, it is not a prime number.