a(1)=1;
a(2)=1*2=2;
a(3)=1*2*3=6;
a(4)=1*2*3*4=24;
a(5)=1*2*3*4*5=120;
a(6)=1*2*3*4*5*6=720;
a(7)=1*2*3*4*5*6*7=5040;
a(8)=1*2*3*4*5*6*7*8=40320;
a(9)=1*2*3*4*5*6*7*8*9=362880;
a(10)=1*2*3*4*5*6*7*8*9-10=362870;
a(11)=1*2*3*4*5*6*7*8*9-10*11=362770;
a(12)=1*2*3*4*5*6*7*8*9-10*11*12=361560;
a(13)=1*2*3*4*5*6*7*8*9-10*11*12*13=345720;
a(14)=1*2*3*4*5*6*7*8*9-10*11*12*13*14=122640;
a(15)=1*2*3*4*5*6*7*8*9-10*11*12*13*14*15=-3240720;
a(16)=1*2*3*4*5*6*7*8*9-10*11*12*13*14*15*16=-57294720;
a(17)=1*2*3*4*5*6*7*8*9-10*11*12*13*14*15*16*17=-979816320;
a(18)=1*2*3*4*5*6*7*8*9-10*11*12*13*14*15*16*17*18=-17642862720;
a(19)=1*2*3*4*5*6*7*8*9-10*11*12*13*14*15*16*17*18+19=-17642862701;等。
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