A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1537536 can be evenly divided by 1 2 3 4 6 7 8 11 12 13 14 16 21 22 24 26 28 32 33 39 42 44 48 52 56 64 66 77 78 84 88 91 96 104 112 128 132 143 154 156 168 176 182 192 208 224 231 256 264 273 286 308 312 336 352 364 384 416 429 448 462 512 528 546 572 616 624 672 704 728 768 832 858 896 924 1,001 1,056 1,092 1,144 1,232 1,248 1,344 1,408 1,456 1,536 1,664 1,716 1,792 1,848 2,002 2,112 2,184 2,288 2,464 2,496 2,688 2,816 2,912 3,003 3,328 3,432 3,584 3,696 4,004 4,224 4,368 4,576 4,928 4,992 5,376 5,632 5,824 6,006 6,656 6,864 7,392 8,008 8,448 8,736 9,152 9,856 9,984 10,752 11,648 12,012 13,728 14,784 16,016 16,896 17,472 18,304 19,712 19,968 23,296 24,024 27,456 29,568 32,032 34,944 36,608 39,424 46,592 48,048 54,912 59,136 64,064 69,888 73,216 96,096 109,824 118,272 128,128 139,776 192,192 219,648 256,256 384,384 512,512 768,768 and 1,537,536, with no remainder.
Since 1,537,536 cannot be divided by just 1 and 1,537,536, it is not a prime number.