A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1577520 can be evenly divided by 1 2 3 4 5 6 7 8 9 10 12 14 15 16 18 20 21 24 28 30 35 36 40 42 45 48 56 60 63 70 72 80 84 90 105 112 120 126 140 144 168 180 210 240 252 280 313 315 336 360 420 504 560 626 630 720 840 939 1,008 1,252 1,260 1,565 1,680 1,878 2,191 2,504 2,520 2,817 3,130 3,756 4,382 4,695 5,008 5,040 5,634 6,260 6,573 7,512 8,764 9,390 10,955 11,268 12,520 13,146 14,085 15,024 17,528 18,780 19,719 21,910 22,536 25,040 26,292 28,170 32,865 35,056 37,560 39,438 43,820 45,072 52,584 56,340 65,730 75,120 78,876 87,640 98,595 105,168 112,680 131,460 157,752 175,280 197,190 225,360 262,920 315,504 394,380 525,840 788,760 and 1,577,520, with no remainder.
Since 1,577,520 cannot be divided by just 1 and 1,577,520, it is not a prime number.