A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2581632 can be evenly divided by 1 2 3 4 6 8 9 12 16 18 24 27 32 36 48 54 64 72 81 83 96 108 128 144 162 166 192 216 243 249 288 324 332 384 432 486 498 576 648 664 747 864 972 996 1,152 1,296 1,328 1,494 1,728 1,944 1,992 2,241 2,592 2,656 2,988 3,456 3,888 3,984 4,482 5,184 5,312 5,976 6,723 7,776 7,968 8,964 10,368 10,624 11,952 13,446 15,552 15,936 17,928 20,169 23,904 26,892 31,104 31,872 35,856 40,338 47,808 53,784 71,712 80,676 95,616 107,568 143,424 161,352 215,136 286,848 322,704 430,272 645,408 860,544 1,290,816 and 2,581,632, with no remainder.
Since 2,581,632 cannot be divided by just 1 and 2,581,632, it is not a prime number.