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====Other notations====
In some countries such as [[Germany]], the above multiplication is depicted similarly but with the original product kept horizontal and computation starting with the first digit of the multiplier:<ref>{{Cite web |title=Multiplication |url=
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====History of quarter square multiplication====
In prehistoric time, quarter square multiplication involved [[Floor and ceiling functions|floor function]]; that some sources<ref>{{citation |title= Quarter Tables Revisited: Earlier Tables, Division of Labor in Table Construction, and Later Implementations in Analog Computers |last=McFarland |first=David|url=
Antoine Voisin published a table of quarter squares from 1 to 1000 in 1817 as an aid in multiplication. A larger table of quarter squares from 1 to 100000 was published by Samuel Laundy in 1856,<ref>{{Citation |title=Reviews |journal=The Civil Engineer and Architect's Journal |year=1857 |pages=54–55 |url=https://books.google.com/books?id=gcNAAAAAcAAJ&pg=PA54 |postscript=.}}</ref> and a table from 1 to 200000 by Joseph Blater in 1888.<ref>{{Citation|title=Multiplying with quarter squares |first=Neville |last=Holmes| journal=The Mathematical Gazette |volume=87 |issue=509 |year=2003 |pages=296–299 |jstor=3621048|postscript=.|doi=10.1017/S0025557200172778 |s2cid=125040256 }}</ref>
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===Lower bounds===
There is a trivial lower bound of [[Big O notation#Family of Bachmann–Landau notations|Ω]](''n'') for multiplying two ''n''-bit numbers on a single processor; no matching algorithm (on conventional machines, that is on Turing equivalent machines) nor any sharper lower bound is known. The [[Hartmanis–Stearns conjecture]] would imply that <math>O(n)</math> cannot be achieved. Multiplication lies outside of [[ACC0|AC<sup>0</sup>[''p'']]] for any prime ''p'', meaning there is no family of constant-depth, polynomial (or even subexponential) size circuits using AND, OR, NOT, and MOD<sub>''p''</sub> gates that can compute a product. This follows from a constant-depth reduction of MOD<sub>''q''</sub> to multiplication.<ref>{{cite book |first1=Sanjeev |last1=Arora |first2=Boaz |last2=Barak |title=Computational Complexity: A Modern Approach |publisher=Cambridge University Press |date=2009 |isbn=978-0-521-42426-4 |url={{GBurl|8Wjqvsoo48MC|pg=PR7}}}}</ref> Lower bounds for multiplication are also known for some classes of [[branching program]]s.<ref>{{cite journal |first1=F. |last1=Ablayev |first2=M. |last2=Karpinski |title=A lower bound for integer multiplication on randomized ordered read-once branching programs |journal=Information and Computation |volume=186 |issue=1 |pages=78–89 |date=2003 |doi=10.1016/S0890-5401(03)00118-4 |url=https://core.ac.uk/download/pdf/82445954.pdf}}</ref>
==Complex number multiplication==
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===Basic arithmetic===
* [
* [
* [
===Advanced algorithms===
* [
{{Number-theoretic algorithms}}
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