a（1）=6，因为丢番图2x+3y=2^3+3^3有6个正解{x，y}：
{1, 11}, {4, 9}, {7, 7}, {10, 5}, {13, 3}, {16, 1};
a（2）=10，因为丢番图3x+5y=3^3+5^3有10个正解{x，y}：
{4, 28}, {9, 25}, {14, 22}, {19, 19}, {24, 16}, {29, 13}, {34, 10}, {39, 7}, {44, 4}, {49, 1};
a（3）=13，因为丢番图5x+7y=5^3+7^3有13个正解{x，y}：
{4, 64}, {11, 59}, {18, 54}, {25, 49}, {32, 44}, {39, 39}, {46, 34}, {53, 29}, {60, 24}, {67, 19}, {74, 14}, {81, 9}, {88, 4}.
