A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 821730 can be evenly divided by 1 2 3 5 6 7 10 13 14 15 21 26 30 35 39 42 43 49 65 70 78 86 91 98 105 129 130 147 182 195 210 215 245 258 273 294 301 390 430 455 490 546 559 602 637 645 735 903 910 1,118 1,274 1,290 1,365 1,470 1,505 1,677 1,806 1,911 2,107 2,730 2,795 3,010 3,185 3,354 3,822 3,913 4,214 4,515 5,590 6,321 6,370 7,826 8,385 9,030 9,555 10,535 11,739 12,642 16,770 19,110 19,565 21,070 23,478 27,391 31,605 39,130 54,782 58,695 63,210 82,173 117,390 136,955 164,346 273,910 410,865 and 821,730, with no remainder.
Since 821,730 cannot be divided by just 1 and 821,730, it is not a prime number.