A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1606836 can be evenly divided by 1 2 3 4 6 7 11 12 14 21 22 28 33 37 42 44 47 66 74 77 84 94 111 132 141 148 154 188 222 231 259 282 308 329 407 444 462 517 518 564 658 777 814 924 987 1,034 1,036 1,221 1,316 1,551 1,554 1,628 1,739 1,974 2,068 2,442 2,849 3,102 3,108 3,478 3,619 3,948 4,884 5,217 5,698 6,204 6,956 7,238 8,547 10,434 10,857 11,396 12,173 14,476 17,094 19,129 20,868 21,714 24,346 34,188 36,519 38,258 43,428 48,692 57,387 73,038 76,516 114,774 133,903 146,076 229,548 267,806 401,709 535,612 803,418 and 1,606,836, with no remainder.
Since 1,606,836 cannot be divided by just 1 and 1,606,836, it is not a prime number.