A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1541592 can be evenly divided by 1 2 3 4 6 8 9 12 13 18 24 26 27 36 39 52 54 61 72 78 81 104 108 117 122 156 162 183 216 234 243 244 312 324 351 366 468 486 488 549 648 702 732 793 936 972 1,053 1,098 1,404 1,464 1,586 1,647 1,944 2,106 2,196 2,379 2,808 3,159 3,172 3,294 4,212 4,392 4,758 4,941 6,318 6,344 6,588 7,137 8,424 9,516 9,882 12,636 13,176 14,274 14,823 19,032 19,764 21,411 25,272 28,548 29,646 39,528 42,822 57,096 59,292 64,233 85,644 118,584 128,466 171,288 192,699 256,932 385,398 513,864 770,796 and 1,541,592, with no remainder.
Since 1,541,592 cannot be divided by just 1 and 1,541,592, it is not a prime number.