91 = 7 * 13.
8191是质数==>a（1）=0。
810901 = 7^2 * 13 * 19 * 67.
81009001是质数==>a（2）=3。
8100090001 = 73 * 271 * 331 * 1237.
810000900001 = 1471 * 550646431.
81000009000001 = 7 * 13 * 613 * 757 * 1129 * 1699.
8100000090000001 = 31 * 439 * 595194363289.
810000000900000001 = 7 * 13 * 157 * 193 * 4243 * 4969 * 13933.
81000000009000000001 = 271 * 298892988963099631.
