A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1589616 can be evenly divided by 1 2 3 4 6 7 8 9 12 14 16 18 19 21 24 28 36 38 42 48 56 57 63 72 76 83 84 112 114 126 133 144 152 166 168 171 228 249 252 266 304 332 336 342 399 456 498 504 532 581 664 684 747 798 912 996 1,008 1,064 1,162 1,197 1,328 1,368 1,494 1,577 1,596 1,743 1,992 2,128 2,324 2,394 2,736 2,988 3,154 3,192 3,486 3,984 4,648 4,731 4,788 5,229 5,976 6,308 6,384 6,972 9,296 9,462 9,576 10,458 11,039 11,952 12,616 13,944 14,193 18,924 19,152 20,916 22,078 25,232 27,888 28,386 33,117 37,848 41,832 44,156 56,772 66,234 75,696 83,664 88,312 99,351 113,544 132,468 176,624 198,702 227,088 264,936 397,404 529,872 794,808 and 1,589,616, with no remainder.
Since 1,589,616 cannot be divided by just 1 and 1,589,616, it is not a prime number.