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This problem illustrates how to do the Fourier Transform (FT) in modular arithmetic, for example, modulo .(a) There is a number such that all the powers ,2,...,6 are distinct (modulo ). Find this role="math" localid="1659339882657" , and show that +2+...+6=0. (Interestingly, for any prime modulus there is such a number.)

(b) Using the matrix form of the FT, produce the transform of the sequence (0,1,1,1,5,2) modulo 7; that is, multiply this vector by the matrix M6(), for the value of you found earlier. In the matrix multiplication, all calculations should be performed modulo 7.

(c) Write down the matrix necessary to perform the inverse FT. Show that multiplying by this matrix returns the original sequence. (Again all arithmetic should be performed modulo 7.)

(d) Now show how to multiply the polynomials and using the FT modulo 7.

Short Answer

Expert verified
  1. =3
  2. Transformation of the sequence (0,1,1,1,5,2)is(3,6,4,2,3,3). .
  3. Transformation of the sequence (3,6,4,2,3,3)is(0,1,1,1,5,2). .
  4. Transformation of the sequence (3,6,4,2,3,3)is(-1,1,1,3,1,1)..

Step by step solution

01

Solution of part (a) and (b)  

a.)

Find the value of :

On observing, the value of =3 鈥︹赌

The powers of w are distinct modulo:

Value of 2:

鈥 Substitute =3 in localid="1659340352169" 2and take modulo .

2=32mod7=9mod7=2

Value of 3:

鈥 Substitute =3in3 and take modulo .

3=27mod7=6

Value of 4:

鈥 Substitute =3in4 and take modulo .

localid="1659340501146" 4=34mod7=81mod7=4

Value of 5:

鈥 Substitute =3in5 and take modulo .

5=35mod7=243mod7=5

Value of 6:

鈥 Substitute =3in6 and take modulo .

6=36mod7=729mod7=1

The values of (=3,2=2,3=6,4=4,5=5,6=1) are distinct.

To show +2+3+4+5+6=0 :

鈥 Substitute the values (=3,2=2,3=6,4=4,5=5,6=1)and take modulo in following:

+2+3+4+5+6(2)

Thus,

(+2+3+4+5+6)mod7=(3+2+6+4+5+1)mod7=21mod7=0

Therefore, +2+3+4+5+6=0

02

Solution of part   (b).  

b.)

Coefficient representation of 66matrix of FFT (Fast Fourier Transform)M6() ,is shown below:

M6()=111111123451246810136912151481216201510152025 (3).....

From section (a) the value of powers of are obtained as:

=3,2=2,3=6,4=4,5=5,6=1 (4) 鈥︹赌

The value of is,

8=38=6561mod7=29=39=19683mod7=610=310=59049mod7=412=312=531441mod7=115=315=14348907mod7=616=316=143046721mod7=420=320=3486784401mod7=225=325=84728809443mod7=3 鈥︹赌 (5)

Substitute the powers of w from equation (4) and equation (5) in equation (3).

localid="1659341811588" M6()=111111132645124124161616142142154623. (6)....

The FFT of (0,1,1,1,5,2)is calculated as follows:

FFT of A=M6()A 鈥︹赌 (7)

Substitute the values M6()from Equation (6) and A as (0,1,1,1,5,2) in Equation .

Multiplying the sequence (0,1,1,1,5,2) with equation (6) .

localid="1659341818991" FFTofA=111111132645124124161616142142154623

=101111111512103021614552102141112542106111611562104121114522105141612532

=104125303131...... (8)

Taking modulo for equation .

10mod714mod725mod730mod732mod731mod7=364233

Therefore, the transformation of the sequence (0,1,1,1,5,2)is(3,6,4,2,3,3)

03

Solution of part (c).  

c.)

Inverse FFT of (3,6,4,2,3,3)::

Let P=(3,6,4,2,3,3) 鈥︹赌 (9)

Coefficient representation of M6((-1)) is shown below:

Inverse FFT(P)=16PM6(-1)鈥︹赌 (10)

To represent the matrix form for the inverse FFT with modulo first find the modular inverse. Modular inverse of,

1mod7=12mod7=43mod7=54mod7=25mod7=36mod7=6(11)

To get localid="1659345943519" M6(-1), substitute the inverse of each values. Thus,

localid="1659346430928" M6(-1)=111111111111111514161213111412111412111611161116111214111214111312161415

Substitute equation (9), equation (12) in equation (10).

localid="1659346499692" InverseFTT(P)=1/6111111111111111514161213111412111412111611161116111214111214111312161415364233鈥︹赌

Thus, substitute the arithmetic value for the arithmetic value 11, for the arithmetic value 1,12, for the arithmetic value 5,14, for the arithmetic value 2,15, for the arithmetic value 3 and 16 for the arithmetic value 6 from equation (11) in equation (13) to find the inverse.

InverseFTT(P)=1/6111111111111111514161213111412111412111611161116111214111214111312161415364233

=6131614121313135644622333134624124323136614621363132644122343133624624353=6217655765168=126456330456306408 鈥︹赌 (14)

Taking modulo 7 for equation ( 14 ).

=126mod7456mod7330mod7446mod7306mod7408mod7=011152

Therefore, the transformation of the sequence (3,6,4,2,3,3)is(0,1,1,1,5,2)..

04

Solution of part (d).  

d.)

The standard form for degree-d polynomial is

A(x)=a0+a1x++adxd(15)

The first polynomial is x2+x+1(16)

Compare equation (15) and (16) equation

a0=1a1=1a2=1a3=0a4=0a5=0(17)

Consider the formula of polynomial to matrix transformation.

localid="1659427656504" a0a1a2a3a4a5 鈥︹赌

Substitute the values of a0,a1,a2,a3,a4,a5 in Equation (18)

A=111000鈥︹赌

Find FFT of (1,1,1,0,0,0)::

Substitute the values M6() from Equation (6) and A from Equation (19) in Equation (7) .

localid="1659418277264" FFTofA=111111132645124124161616142142154623111000=11+11+11+10+10+1011+31+21+60+40+5011+21+41+10+20+4011+61+11+60+10+6011+41+21+10+40+2011+51+41+60+20+30=3278710......20

Taking modulo 7 for the FFT of equation (20) .

3mod76mod77mod78mod77mod710mod7=360103

Therefore, the transformation of the sequence (1,1,1,0,0,0)is(3,6,0,1,0,3)

The second polynomial is x3+2x-1鈥︹赌

Compare equations (21) and equation (15)

a0=-1a1=2a2=0a3=1a4=0a5=0(22)

The value of is so take modulo as shown below:

-1mod7=6(23)

Thus,a0=-1 substitute a0 as 6 in equations (22)

a0=6a1=2a2=0a3=1a4=0a5=0

Consider the formula of polynomial to matrix transformation.

B=a0a1a2a3a4a5.....24

Substitute the values of a0,a1,a2,a3,a4,a5 in Equation (24) .

B=620100.....25

Find FFT of (6,2,0,1,0,0):

Substitute the values M6()from Equation (6) and B from Equation (25) in Equation (7) .

FFTofB=111111132645124124161616142142154623620100=161210111010163220614050162240112040166210611060164220114020165210612030=91811241522.....26

Taking modulo 7 for equation (26)

=9mod718mod711mod724mod715mod722mod7=244311

Therefore, the transformation of the sequence (6,2,0,1,0,0)is(2,4,4,3,1,1)

Let the product of FFT of A and B be P. The product is calculated as shown below:

P=AB(27)

Substitute the value of 鈥 A鈥 from Equation (19) and 鈥淏 鈥 from Equation (25) in Equation(27) .

P=360103244311=630303....28 鈥︹赌

Modular inverse of,

6mod7=624mod7=240mod7=03mod7=30mod7=03mod7=3....29

Substitute the Equation in Equation . Thus, the product is shown below:

P=630303....30

Inverse FFT:

To get the final polynomial in coefficient representation takes the inverse FFT for the product.

Substitute equation (30) , equation (12) in equation (10) .

InverseFTT(P)=16111111111111111514161213111412111412111611161116111214111214111312161415630303......31

Thus, substitute the arithmetic value 11 for the arithmetic value 1, 12 for the arithmetic value 4,13 for the arithmetic value 5,14 for the arithmetic value 2,15 for the arithmetic value 3 and 16 for the arithmetic value 6 from equation in(11) equation (13) to find the inverse.

localid="1659426193884" InverseFTT(P)=6111111111111111514161213111412111412111611161116111214111214111312161415630303=616+13+10+13+10+1316+33+20+63+40+5316+23+40+13+20+4316+63+10+63+10+6316+43+20+13+40+2316+53+40+63+20+33=6124827622748=90288162360162288......32

Taking modulo for equation .

=90mod7288mod7162mod7360mod7162mod7288mod7=611311......30

From equation (23) substitute 6 as -1 in equation (33)

InverseFFTof(P)=-111311

Therefore, the transformation of the sequence (3,6,4,2,3,3)is(-1,1,1,3,1,1). .

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Most popular questions from this chapter

You are given an array of nelements, and you notice that some of the elements are duplicates; that is, they appear more than once in the array. Show how to remove all duplicates from the array in time O(nlogn) .

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Thesquare of a matrix A is its product with itself, AA.

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(b) What is wrong with the following algorithm for computing the square of an n x n matrix?

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