Easy way to remember Strassen’s Matrix Equation

Strassen’s matrix is a Divide and Conquer method that helps us to multiply two matrices(of size n X n).

You can refer to the link, for having the knowledge about Strassen’s Matrix first :
Divide and Conquer | Set 5 (Strassen’s Matrix Multiplication)

But this method needs to cram few equations, so I’ll tell you the simplest way to remember those :
stressen_formula_new_new

You just need to remember 4 Rules :

  • AHED (Learn it as ‘Ahead’)
  • Diagnol
  • Last CR
  • First CR

Also, consider X as (Row +) and Y as (Column -) matrix

Follow the Steps :

  • Write P1 = A; P2 = H; P3 = E; P4 = D
  • For P5 we will use Diagnol Rule i.e.
    (Sum the Diagnol Elements Of Matrix X ) * (Sum the Diagnol Elements Of Matrix Y ), we get
    P5 = (A + D)* (E + H)
  • S1

  • For P6 we will use Last CR Rule i.e. Last Column of X and Last Row of Y and remember that Row+ and Column- so i.e. (B – D) * (G + H), we get
    P6 = (B – D) * (G + H)
  • For P7 we will use First CR Rule i.e. First Column of X and First Row of Y and remember that Row+ and Column- so i.e. (A – C) * (E + F), we get
    P6 = (A – C) * (E + F)
  • S2

  • Come Back to P1 : we have A there and it’s adjacent element in Y Matrix is E, since Y is Column Matrix so we select a column in Y such that E won’t come, we find F H Column, so multiply A with (F – H)
    So, finally P1 = A * (F – H)
  • S3

  • Come Back to P2 : we have H there and it’s adjacent element in X Matrix is D, since X is Row Matrix so we select a Row in X such that D won’t come, we find A B Column, so multiply H with (A + B)
    So, finally P2 = H * (A + B)
  • Come Back to P3 : we have E there and it’s adjacent element in X Matrix is A, since X is Row Matrix so we select a Row in X such that A won’t come, we find C D Column, so multiply E with (C + D)
    So, finally P3 = E * (C + D)
  • S4

  • Come Back to P4 : we have D there and it’s adjacent element in Y Matrix is H, since Y is Column Matrix so we select a column in Y such that H won’t come, we find G E Column, so multiply D with (G – E)
    So, finally P4 = D * (G – E)
  • We are done with P1 – P7 equations, so now we move to C1 – C4 equations in Final Matrix C :

  • Remember Counting : Write P1 + P2 at C2
  • Write P3 + P4 at its diagnol Position i.e. at C3
  • Write P4 + P5 + P6 at 1st position and subtract P2 i.e. C1 = P4 + P5 + P6 – P2
  • Write odd values at last Position with alternating – and + sign i.e. P1 P3 P5 P7 becomes
    C4 = P1 – P3 + P5 – P7
  • S5

This article is contributed by Mohit Gupta 🙂. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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