A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1636488 can be evenly divided by 1 2 3 4 6 7 8 9 12 14 17 18 21 24 28 34 36 42 51 56 63 68 72 84 102 119 126 136 153 168 191 204 238 252 306 357 382 408 476 504 573 612 714 764 952 1,071 1,146 1,224 1,337 1,428 1,528 1,719 2,142 2,292 2,674 2,856 3,247 3,438 4,011 4,284 4,584 5,348 6,494 6,876 8,022 8,568 9,741 10,696 12,033 12,988 13,752 16,044 19,482 22,729 24,066 25,976 29,223 32,088 38,964 45,458 48,132 58,446 68,187 77,928 90,916 96,264 116,892 136,374 181,832 204,561 233,784 272,748 409,122 545,496 818,244 and 1,636,488, with no remainder.
Since 1,636,488 cannot be divided by just 1 and 1,636,488, it is not a prime number.