A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1598016 can be evenly divided by 1 2 3 4 6 7 8 12 14 16 21 24 28 29 32 41 42 48 56 58 64 82 84 87 96 112 116 123 164 168 174 192 203 224 232 246 287 328 336 348 406 448 464 492 574 609 656 672 696 812 861 928 984 1,148 1,189 1,218 1,312 1,344 1,392 1,624 1,722 1,856 1,968 2,296 2,378 2,436 2,624 2,784 3,248 3,444 3,567 3,936 4,592 4,756 4,872 5,568 6,496 6,888 7,134 7,872 8,323 9,184 9,512 9,744 12,992 13,776 14,268 16,646 18,368 19,024 19,488 24,969 27,552 28,536 33,292 38,048 38,976 49,938 55,104 57,072 66,584 76,096 99,876 114,144 133,168 199,752 228,288 266,336 399,504 532,672 799,008 and 1,598,016, with no remainder.
Since 1,598,016 cannot be divided by just 1 and 1,598,016, it is not a prime number.