<p>The <strong class="pkg">GAP</strong> category of <strong class="pkg">homalg</strong> bigraded objects associated to a filtered complex. <br /> The <span class="SimpleMath">\(0\)</span>-th spectral sheet <span class="SimpleMath">\(E_0\)</span> stemming from a filtration is a bigraded (differential) object, which, in general, does not stem from an exact couple (although <span class="SimpleMath">\(E_1\)</span>, <span class="SimpleMath">\(E_2\)</span>, ... do).</p>
<p>(It is a subcategory of the <strong class="pkg">GAP</strong> category <code class="code">IsHomalgBigradedObject</code>.)</p>
<p>The <strong class="pkg">GAP</strong> category of <strong class="pkg">homalg</strong> bigraded objects associated to a bicmplex.</p>
<p>(It is a subcategory of the <strong class="pkg">GAP</strong> category <br /> <code class="code">IsHomalgBigradedObjectAssociatedToAFilteredComplex</code>.)</p>
<p>The <strong class="pkg">GAP</strong> representation of bigraded objects of finitley generated <strong class="pkg">homalg</strong> objects.</p>
<p>(It is a representation of the <strong class="pkg">GAP</strong> category <code class="func">IsHomalgBigradedObject</code> (<a href="chap9_mj.html#X795C082E83748032"><span class="RefLink">9.1-1</span></a>), which is a subrepresentation of the <strong class="pkg">GAP</strong> representation <code class="code">IsFinitelyPresentedObjectRep</code>.)</p>
<p>This constructor creates a homological (resp. cohomological) bigraded object given a homological (resp. cohomological) <strong class="pkg">homalg</strong> bicomplex <var class="Arg">B</var> (--> <code class="func">HomalgBicomplex</code> (<a href="chap8_mj.html#X86D50FE285F49BF6"><span class="RefLink">8.2-1</span></a>)). This is nothing but the level zero sheet (without differential) of the spectral sequence associated to the bicomplex <var class="Arg">B</var>. So it is the double array of <strong class="pkg">homalg</strong> objects (i.e. static objects or complexes) in <var class="Arg">B</var> forgetting the morphisms.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">zz := HomalgRingOfIntegers( );</span>
Z
<span class="GAPprompt">gap></span> <span class="GAPinput">M := HomalgMatrix( "[ 2, 3, 4, 5, 6, 7 ]", 2, 3, zz );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">M := LeftPresentation( M );</span>
<A non-torsion left module presented by 2 relations for 3 generators>
<span class="GAPprompt">gap></span> <span class="GAPinput">d := Resolution( M );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">dd := Hom( d );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">C := Resolution( dd );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">CC := Hom( C );</span>
<A non-zero acyclic complex containing a single morphism of left cocomplexes a\
t degrees [ 0 .. 1 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">B := HomalgBicomplex( CC );</span>
<A non-zero bicomplex containing left modules at bidegrees [ 0 .. 1 ]x
[ -1 .. 0 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">E0 := HomalgBigradedObject( B );</span>
<A bigraded object containing left modules at bidegrees [ 0 .. 1 ]x
[ -1 .. 0 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( E0 );</span>
Level 0:
<p>Add the induced bidegree <span class="SimpleMath">\(( -r, r - 1 )\)</span> (resp. <span class="SimpleMath">\(( r, -r + 1 )\)</span>) differential to the level <var class="Arg">r</var> homological (resp. cohomological) bigraded object stemming from a homological (resp. cohomological) bicomplex. This method performs side effects on its argument <var class="Arg">Er</var> and returns it.</p>
<p>For an example see <code class="func">DefectOfExactness</code> (<a href="chap9_mj.html#X783AA6E3817BFC0F"><span class="RefLink">9.2-3</span></a>) below.</p>
<p>Homological: Compute the homology of a level <var class="Arg">r</var> <em>differential</em> homological bigraded object, that is the <var class="Arg">r</var>-th sheet of a homological spectral sequence endowed with a bidegree <span class="SimpleMath">\(( -r, r - 1 )\)</span> differential. The result is a level <var class="Arg">r</var><span class="SimpleMath">\(+1\)</span> homological bigraded object <em>without</em> its differential.</p>
<p>Cohomological: Compute the cohomology of a level <var class="Arg">r</var> <em>differential</em> cohomological bigraded object, that is the <var class="Arg">r</var>-th sheet of a cohomological spectral sequence endowed with a bidegree <span class="SimpleMath">\(( r, -r + 1 )\)</span> differential. The result is a level <var class="Arg">r</var><span class="SimpleMath">\(+1\)</span> cohomological bigraded object <em>without</em> its differential.</p>
<p>The differential of the resulting level <var class="Arg">r</var><span class="SimpleMath">\(+1\)</span> object can a posteriori be computed using <code class="func">AsDifferentialObject</code> (<a href="chap9_mj.html#X7D0A240684BD8FC3"><span class="RefLink">9.2-2</span></a>). The objects in the result are subquotients of the objects in <var class="Arg">Er</var>. An object in <var class="Arg">Er</var> (at a spot <span class="SimpleMath">\((p,q)\)</span>) is called <em>stable</em> if no passage to a true subquotient occurs at any higher level. Of course, a zero object (at a spot <spanclass="SimpleMath">\((p,q)\)</span>) is always stable.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">zz := HomalgRingOfIntegers( );</span>
Z
<span class="GAPprompt">gap></span> <span class="GAPinput">M := HomalgMatrix( "[ 2, 3, 4, 5, 6, 7 ]", 2, 3, zz );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">M := LeftPresentation( M );</span>
<A non-torsion left module presented by 2 relations for 3 generators>
<span class="GAPprompt">gap></span> <span class="GAPinput">d := Resolution( M );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">dd := Hom( d );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">C := Resolution( dd );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">CC := Hom( C );</span>
<A non-zero acyclic complex containing a single morphism of left cocomplexes a\
t degrees [ 0 .. 1 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">B := HomalgBicomplex( CC );</span>
<A non-zero bicomplex containing left modules at bidegrees [ 0 .. 1 ]x
[ -1 .. 0 ]>
</pre></div>
<p>Now we construct the spectral sequence associated to the bicomplex <span class="SimpleMath">\(B\)</span>, also called the <em>first</em> spectral sequence:</p>
* *
* *
<span class="GAPprompt">gap></span> <span class="GAPinput">AsDifferentialObject( I_E0 );</span>
<A bigraded object with a differential of bidegree
[ 0, -1 ] containing left modules at bidegrees [ 0 .. 1 ]x[ -1 .. 0 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">I_E0;</span>
<A bigraded object with a differential of bidegree
[ 0, -1 ] containing left modules at bidegrees [ 0 .. 1 ]x[ -1 .. 0 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">AsDifferentialObject( I_E0 );</span>
<A bigraded object with a differential of bidegree
[ 0, -1 ] containing left modules at bidegrees [ 0 .. 1 ]x[ -1 .. 0 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">I_E1 := DefectOfExactness( I_E0 );</span>
<A bigraded object containing left modules at bidegrees [ 0 .. 1 ]x
[ -1 .. 0 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( I_E1 );</span>
Level 1:
* *
. .
<span class="GAPprompt">gap></span> <span class="GAPinput">AsDifferentialObject( I_E1 );</span>
<A bigraded object with a differential of bidegree
[ -1, 0 ] containing left modules at bidegrees [ 0 .. 1 ]x[ -1 .. 0 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">I_E2 := DefectOfExactness( I_E1 );</span>
<A bigraded object containing left modules at bidegrees [ 0 .. 1 ]x
[ -1 .. 0 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( I_E2 );</span>
Level 2:
s .
. .
</pre></div>
<p>Legend:</p>
<ul>
<li><p>A star <var class="Arg">*</var> stands for a nonzero object.</p>
</li>
<li><p>A dot <var class="Arg">.</var> stands for a zero object.</p>
</li>
<li><p>The letter <var class="Arg">s</var> stands for a nonzero object that became stable.</p>
</li>
</ul>
<p>The <em>second</em> spectral sequence of the bicomplex is, by definition, the spectral sequence associated to the transposed bicomplex:</p>
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