Ĺ L
(xm,ym,h)=½[f(xm,ym)+f(xm+h,ym+y¢mh)] 1.2
y¢m=f(xm,ym) 1.3
L
y=ym+(x-xm)(xm,ym,h),
ym+1=ym+h(xm,ym,h). 1.4
1.2, 1.3, 1.4 .
, , , f(x,y) :
f(x,y)=f(xm,ym)+(x-xm)f/x+(y-ym)f/x+¼ 1.5
x=xm y=ym.
1.5 x=xm+h y=ym+hy¢m 1.3 y¢m,
f(xm+h,ym+hy¢m)=f+hfx+hffy+O(h2),
f xm,ym. 1.2 ,
(xm,ym,h)=f+h/2(fx+ffy)+O(h2).
1.4
ym+1=ym+hf+h2/2(fx+ffy)+O(h3).
, h2, , , - .
. .3 , .2. , x=x+h/2. , y=ym+(h/2)y¢m.
(xm,ym,h)=f+(xm+h/2,ym+h/2*y¢m), 1.6
y¢m=f(xm,ym) 1.7
, , L*. , xm,ym L*, L0. x=xm+h xm+1,ym+1. y=ym+(x-xm)(xm,ym,h),
1.6.
ym+1=ym+h(xm,ym,h) 1.8
1.6, 1.7, 1.8 - . . ,
ym+1=ym+h(xm,ym,h) 1.9
(xm,ym,h)=a1f(xm,ym)+a2f(xm+b1h,ym+b2hy¢m), 1.10
y¢m=f(xm,ym) 1.11
,
a1=a2=1/2;
b1=b2=1.
nbsp;
a1=0, a2=1,
b1=b2=1/2.
1.9, 1.10, 1.11 -. , a1, a2, b1 b2.
|
|
h, . h2, , h2fx h2ffy. , h2, , - .
f(x,y) 1.5 xm,ym x=xm+b1h,
y=ym+b2hf.
f(xm+b1h,ym+b2hf)=f+b1hfx+b2hffy+O(h2), xm,ym.
1.9 ym+1=ym+h[a1f+a2f+h(a2b1fx+a2b2ffy)]+O(h3).
,
ym+1=ym+h[a1f+a2f+h(a2b1fx+a2b2ffy)]+O(h3).
hf, a1+a2=1.
, h2fx, a2b1=1/2.
, h2ffy, a2b2=1/2.
, , , , , , .
, , a2=w¹0. a1=1-w, b1=b2=1/2w 1.9, 1.10, 1.11
ym+1=ym+h[(1-w)f(xm,ym)+wf(xm+h/2w,ym+h/2wf(xm,ym))]+O(h3) 1.12
- . w=1/2 , w=1 . w, ,
et=kh3 1.13
- , . , , , , . -
ym+1=ym+h/6(R1+2R2+2R3+R4) 1.14
R1=f(xm,ym), 1.15
R2=f(xm+h/2,ym+hR1/2), 1.16
R3=f(xm+h/2,ym+hR2/2), 1.17
R4=f(xm+h/2,ym+hR3/2). 1.18
et=kh5
1.14-1.18 . , .