A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1265550 can be evenly divided by 1 2 3 5 6 10 11 13 15 22 25 26 30 33 39 50 55 59 65 66 75 78 110 118 130 143 150 165 177 195 275 286 295 325 330 354 390 429 550 590 649 650 715 767 825 858 885 975 1,298 1,430 1,475 1,534 1,650 1,770 1,947 1,950 2,145 2,301 2,950 3,245 3,575 3,835 3,894 4,290 4,425 4,602 6,490 7,150 7,670 8,437 8,850 9,735 10,725 11,505 16,225 16,874 19,175 19,470 21,450 23,010 25,311 32,450 38,350 42,185 48,675 50,622 57,525 84,370 97,350 115,050 126,555 210,925 253,110 421,850 632,775 and 1,265,550, with no remainder.
Since 1,265,550 cannot be divided by just 1 and 1,265,550, it is not a prime number.