# Trachtenberg System

Contents

The Trachtenberg System is a system of rapid mental calculation. The system consists of a number of readily memorized operations that allow one to perform arithmetic computations very quickly. It was developed by Jakow Trachtenberg in order to keep his mind occupied while being held in a Nazi concentration camp.

## General Multiplication

The method for general multiplication is a method to achieve multiplications a*b with low space complexity, i.e. as few temporary results as possible to be kept in memory.

This is achieved by noting that the final digit is completely determined by multiplying the last digit of the multiplicands. This is held as a temporary result. To find the next to last digit, we need everything that influences this digit: The temporary result, the last digit of a times the next-to-last digit of b, as well as the next-to-last digit of a times the last digit of b. This calculation is performed, and we have a temporary result that is correct in the final two digits.

In general, for each position n in the final result, we sum for all i:

[a \mbox{ (digit at } i\mbox{)} \times b \mbox{ (digit at } (n-i)\mbox{)} ]

Ordinary people can learn this algorithm and thus multiply four digit numbers in their head - writing down only the final result. They would write it out starting with the rightmost digit and finishing with the leftmost.

Trachtenberg defined this algorithm with a kind of pairwise multiplication where two digits are multiplied by one digit, essentially only keeping the middle digit of the result. By performing the above algorithm with this pairwise multiplication, even fewer temporary results need to be held.

Example: 123456 * 789

To find the first digit of the answer:
The units digit of 9 * 6 = 4.

To find the second digit of the answer, start at the second digit of the multiplicand:
The units digit of 9 5 plus the tens digit of 9 6 plus
The units digit of 8 * 6.
5 + 5 + 8 = 18.
The second digit of the answer is 8 and carry 1 to the third digit.

To find the fourth digit of the answer, start at the fourth digit of the multiplicand:
The units digit of 9 3 plus the tens digit of 9 4 plus
The units digit of 8 4 plus the tens digit of 8 5 plus
The units digit of 7 5 plus the tens digit of 7 6.
7 + 3 + 2 + 4 + 5 + 4 = 25 + 1 carried from the third digit.
The fourth digit of the answer is 6 and carry 2 to the next digit.

## General division

A method of adding columns of numbers and accurately checking the result without repeating the first operation. An intermediate sum, in the form of two rows of digits, is produced. The answer is obtained by taking the sum of the intermediate results with an L-shaped algorithm. As a final step, the checking method that is advocated removes both the risk of repeating any original errors and allows the precise column in which an error occurs to be identified at once. It is based on a check (or digit) sums, such as the nines-remainder method.

For the procedure to be effective, the different operations used in each stages must be kept distinct, otherwise there is a risk of interference.

## Other multiplication algorithms

When performing any of these multiplication algorithms the following “steps” should be applied.

The answer must be found one digit at a time starting at the least significant digit and moving left. The last calculation is on the leading zero of the multiplicand.

Each digit has a neighbor, i.e., the digit on its right. The rightmost digit’s neighbor is the trailing zero.

The ‘halve’ operation has a particular meaning to the Trachtenberg system. It is intended to mean “half the digit, rounded down” but for speed reasons people following the Trachtenberg system are encouraged to make this halving process instantaneous. So instead of thinking “half of seven is three and a half, so three” it’s suggested that one thinks “seven, three”. This speeds up calculation considerably. In this same way the tables for subtracting digits from 10 or 9 are to be memorized.

And whenever the rule calls for adding half of the neighbor, always add 5 if the current digit is odd. This makes up for dropping 0.5 in the next digit’s calculation.

### Multiplying by 11

Rule: Add the digit to its neighbor. (By “neighbor” we mean the digit on the right.)

Example: 3,425 * 11 = 37,675

3 7 6 7 5

(=0+3) (=3+4) (=4+2) (=2+5) (=5+0)

To illustrate:
11=10+1

Thus,

[ 3425 \times 11 = 3425 \times (10+1) ] [ \Rightarrow 37675 = 34250 + 3425 ]

### Multiplying by 12

Rule: to multiply by 12:
Starting from the rightmost digit, double each digit and add the neighbor. (By “neighbor” we mean the digit on the right.)

If the answer is greater than a single digit, simply carry over the extra digit (which will be a 1 or 2) to the next operation. The remaining digit is one digit of the final result.

Example: 316 * 12

Determine neighbors in the multiplicand 0316:

• digit 6 has no right neighbor

• digit 1 has neighbor 6

• digit 3 has neighbor 1

• digit 0 (the prefixed zero) has neighbor 3

6 2 = 12 (2 carry 1)
1
2 + 6 + 1 = 9
3 2 + 1 = 7
0
2 + 3 = 3
0 * 2 + 0 = 0

316 * 12 = 3,792

### Multiplying by 6

Rule: to multiply by 6: Add half of the neighbor to each digit, then, if the current digit is odd, add 5.

Example:
357 × 6 = 2142

Working right to left,
7 has no neighbor, add 5 (since 7 is odd) = 12. Write 2, carry the 1.
5 + half of 7 (3) + 5 (since the starting digit 5 is odd) + 1 (carried) = 14. Write 4, carry the 1.
3 + half of 5 (2) + 5 (since 3 is odd) + 1 (carried) = 11. Write 1, carry 1.
0 + half of 3 (1) + 1 (carried) = 2. Write 2.

### Multiplying by 7

Rule: to multiply by 7: #Double each digit. #Add half of its neighbor. #If the digit is odd, add 5.

Example: 523 x 7 = 3,661.

3x2+0+5=11, 1.
2x2+1+1=6.
5x2+1+5=16, 6.
0x2+2+1=3.

### Multiplying by 9

Rule: #Subtract the right-most digit from 10. ##Subtract the remaining digits from 9. #Add the neighbor. #For the leading zero, subtract 1 from the neighbor.

For rules 9, 8, 4, and 3 only the first digit is subtracted from 10. After that each digit is subtracted from nine instead.

Example: 2,130 × 9 = 19,170

Working from right to left:

(10 - 0) + 0 = 10. Write 0, carry 1. (9 - 3) + 0 + 1 (carried) = 7. Write 7. (9 - 1) + 3 = 11. Write 1, carry 1. (9 - 2) + 1 + 1 (carried) = 9. Write 9. *2 - 1 = 1. Write 1.

### Multiplying by 8

Rule:

1. Subtract right-most digit from 10.
2. #Subtract the remaining digits from 9.
3. Double the result.
5. For the leading zero, subtract 2 from the neighbor.

Example: 456 x 8 = 3648

Working from right to left: (10 - 6) x 2 + 0 = 8. Write 8. (9 - 5) x 2 + 6 = 14, Write 4, carry 1. (9 - 4) x 2 + 5 + 1 (carried) = 16. Write 6, carry 1. 4 - 2 + 1 (carried) = 3. Write 3.

### Multiplying by 4

Rule:

• Subtract the right-most digit from 10.
• Subtract the remaining digits from 9.
• Add half of the neighbor, plus 5 if the digit is odd.
• For the leading 0, subtract 1 from half of the neighbor.

Example: 346 * 4 = 1384

Working from right to left:

(10 - 6) + Half of 0 (0) = 4. Write 4. (9 - 4) + Half of 6 (3) = 8. Write 8. (9 - 3) + Half of 4 (2) + 5 (since 3 is odd) = 13. Write 3, carry 1. Half of 3 (1) - 1 + 1 (carried) = 1. Write 1.

### Multiplying by 3

Rule:

• Subtract the rightmost digit from 10.
• Subtract the remaining digits from 9.
• Double the result.
• Add half of the neighbor, plus 5 if the digit is odd.
• For the leading zero, subtract 2 from half of the neighbor.

Example: 492 x 3 = 1476

Working from right to left: (10 - 2) x 2 + Half of 0 (0) = 16. Write 6, carry 1. (9 - 9) x 2 + Half of 2 (1) + 5 (since 9 is odd) + 1 (carried) = 7. Write 7. (9 - 4) x 2 + Half of 9 (4) = 14. Write 4, carry 1. Half of 4 (2) - 2 + 1 (carried) = 1. Write 1.

### Multiplying by 5

Rule: to multiply by 5: Take half of the neighbor, then, if the current digit is odd, add 5.

Example:

42x5=210
Half of 2’s neighbor, the trailing zero, is 0.
Half of 4’s neighbor is 1.
Half of the leading zero’s neighbor is 2.
43x5=215
Half of 3’s neighbor is 0, plus 5 because 3 is odd, is 5.
Half of 4’s neighbor is 1.
Half of the leading zero’s neighbor is 2.
93x5=465
Half of 3’s neighbor is 0, plus 5 because 3 is odd, is 5.
Half of 9’s neighbor is 1, plus 5 because 9 is odd, is 6.
Half of the leading zero’s neighbor is 4.

## Publications

• Rushan Ziatdinov, Sajid Musa. Rapid mental computation system as a tool for algorithmic thinking of elementary school students development. European Researcher 25(7): 1105-1110, 2012 [http://erjournal.ru/journals_n/1342467174.pdf].
• The Trachtenberg Speed System of Basic Mathematics by Jakow Trachtenberg, A. Cutler (Translator), R. McShane (Translator), was published by Doubleday and Company, Inc. Garden City, New York in 1960. The book contains specific algebraic explanations for each of the above operations.

The algorithms/operations for multiplication etc. can be expressed in other more compact ways that the book doesn’t specify, despite the chapter on algebraic description.

All of this information is from an original book published and printed in 1960. The original book has seven full Chapters and is exactly 270 pages long.

The Chapter Titles are as follows (the numerous sub-categories in each chapter are not listed).

The Trachtenberg speed system of basic mathematics

• Chapter 1 Tables or no tables
• Chapter 2 Rapid multiplication by the direct method
• Chapter 3 Speed multiplication-”two-finger” method
• Chapter 5 Division - Speed and accuracy
• Chapter 6 Squares and square roots
• Chapter 7 Algebraic description of the method

“A revolutionary new method for high-speed multiplication, division, addition, subtraction and square root.” (1960) “The best selling method for high-speed multiplication, division, addition, subtraction and square root - without a calculator.” (Reprinted 2009)

Jakow Trachtenberg (its founder) escaped from Hitler’s Germany from an active institution toward the close of WWII. Professor Trachtenberg fled to Germany when the czarist regime was overthrown in his homeland Russia and lived there peacefully until his mid-thirties when his anti-Hitler attitudes forced him to flee again. He was a fugitive and when captured spent a total of seven years in various concentration camps. It was during these years that Professor Trachtenberg devised the system of speed mathematics. Most of his work was done without pen or paper. Therefore most of the techniques can be performed mentally.