Singular Manual: regIdeal

| #### D.4.16.1 regIdeal Procedure from library `mregular.lib` (see mregular_lib). Usage: regIdeal (i[,e]); i ideal, e integer Return: an integer, the Castelnuovo-Mumford regularity of i. (returns -1 if i is not homogeneous) Assume: i is a homogeneous ideal of the basering S=K[x(0)..x(n)]. e=0: (default) If K is an infinite field, makes random changes of coordinates. If K is a finite field, works over a transcendental extension. e=1: Makes random changes of coordinates even when K is finite. It works if it terminates, but may result in an infinite loop. After 30 loops, a warning message is displayed and -1 is returned. Note: If printlevel > 0 (default = 0), additional info is displayed: dim(S/i), depth(S/i) and end(H^(depth(S/i))(S/i)) are computed, and an upper bound for the a-invariant of S/i is given. The algorithm also determines whether the regularity is attained or not at the last step of a minimal graded free resolution of i, and if the answer is positive, the regularity of the Hilbert function of S/i is given. Example:
    LIB "mregular.lib";
    ring r=0,(x,y,z,t,w),dp;
    ideal i=y2t,x2y-x2z+yt2,x2y2,xyztw,x3z2,y5+xz3w-x2zw2,x7-yt2w4;
    regIdeal(i);
    ==> 10
    regIdeal(lead(std(i)));
    ==> 13
    // Additional information is displayed if you change printlevel (=1);

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