Professor Knopfloch sorgt mal wieder für allgemeine Verwirrung. Je nach Tageslaune schreibt er

1. ${\displaystyle {}{\left(\sum _{i=0}^{k}a_{i}10^{i}\right)}{\left(\sum _{j=0}^{\ell }b_{j}10^{j}\right)}=\sum _{s=0}^{k+\ell }{\left(\sum _{i+j=s}a_{i}b_{j}\right)}10^{s}\,,}$

2. ${\displaystyle {}{\left(\sum _{i=0}^{k}a_{i}10^{i}\right)}{\left(\sum _{j=0}^{\ell }b_{j}10^{j}\right)}=\sum _{s=0}^{k+\ell }{\left(\sum _{i=0}^{k}a_{i}b_{s-i}\right)}10^{s}\,,}$

3. ${\displaystyle {}{\left(\sum _{i=0}^{k}a_{i}10^{i}\right)}{\left(\sum _{j=0}^{\ell }b_{j}10^{j}\right)}=\sum _{s=0}^{k+\ell }{\left(\sum _{i={\max {\left(s-\ell ,0\right)}}}^{\min {\left(k,s\right)}}a_{i}b_{s-i}\right)}10^{s}\,.}$