形如这样的矩阵就叫三对角矩阵
<WBR> |* <WBR> * <WBR> 0 <WBR> <WBR> 0 <WBR> <WBR> 0 | <WBR>
<WBR> |* <WBR> * <WBR> * <WBR> 0 <WBR> <WBR> 0 | <WBR>
<WBR> |0 <WBR> * <WBR> * <WBR> * <WBR> 0 | <WBR>
<WBR> |0 <WBR> <WBR> 0 <WBR> * <WBR> * <WBR> * | <WBR><WBR>
<WBR> |0 <WBR> <WBR> 0 <WBR> <WBR> 0 <WBR> * <WBR> * | <WBR>
星号是数据，其他为零。
以按行为主序的原则转存为一维数组M[k]中，则A[i,j]的对应关系为
k=2*i+j-2.
另一种计算方式为
当i=j+1时k=3*i-3
当i=j时k=3*i-2
当j=i+1时k=3*i-1