modinv(x, N)

Computes multiplicative inverse of x modulo N

Description

Using variable precision integers, computes y such that
mod(x*y, N) == 1 is TRUE.

• MATLAB release: MATLAB 7.2 (R2006a)
• Other requirements: John D'Errico's VPI suite of files.

[John D'Errico]

8-)

[fisher]

It seems the function is not completed. Some of the sub functions are not defined.

[Steven Gregory]

This module, and any others tagged vpi, require John D'Errico's vpi suite of programs. As listed under "Other requirements".

[Nanthini]

hi i used your modinv function the program is given below

a= input('Please input a vpi number: ','s');
f=vpi(a);
e=nextprime(f);
d=modinv(vpi(e),vpi('10641850662478050775300838063179295326495311244216495264720240475083278486076641392553755151560108920715482251286670052918931640724590870256900164694640700350819991051744179236211734410573176997209650364459122853571059016377162766023444746540824998138258407652750335008405190180074534476000047130911251907798499684299595857553973251414375363400538217657736630155539478633941291033068252991757356109381559827851766572043021641234224646877436996703749498922980940934823068060307119029571082281451848485625212652649192958086070122442905758802027526434401481214679307640378414874445695431164943304505187971133265334792817439396071458544945483012899852152133643802968883876'));
disp(d);

For this i got output like this
Please input a vpi number: 78932798709875897985798239632962
  -521510991024643651399669163491559260313526227617474791885763042505857
664977385561168456997539245033945323802087603744630561104830663196609729
027089135771997781645557505192562664968792519121732948128932325159571954
569243970932264987495930313006348655908194700156297275119605911309222114
908823626023733986115222495783413651229506339868566939895630365025986535
210301856596816873363852149927397763766351293662529444692448582845757980
384646334165589784970212497170690948718484808254845836093034729679728997
842249100410667287038983219348274168413156054636466230079069467248343347
119255305695342572748941360728235707527909213139658151308501287396735563
0404723105668513063513

i got negative value for variable 'd'....
The result should be in positive but the result displayed by this function is negative please suggest some idea to rectify this error....

[Steven Gregory]

It's not an error. The number returned is y such that mod(x*y, N) == 1 is TRUE. If you want it to be a positive number, then add N to it.

[Nanthini]

thanks Steven...i got it...