diff --git a/thesis/notes/A_K_Quillen_Pair.tex b/thesis/notes/A_K_Quillen_Pair.tex index afce658..72748c8 100644 --- a/thesis/notes/A_K_Quillen_Pair.tex +++ b/thesis/notes/A_K_Quillen_Pair.tex @@ -1,6 +1,7 @@ -\chapter[A and K form a Quillen pair]{$A$ and $K$ form a Quillen pair} -\label{sec:a-k-quillen-pair} +\chapter{The main equivalence} + +In this section we aim to prove that the homotopy theory of rational spaces is the same as the homootopy theory of cdga's over $\Q$. Before we prove the equivalence, we will show that $A$ and $K$ form a Quillen pair. This already provides an adjunction between the homotopy categories. Besides the equivalence of the homotopy categories we will also investigate homotopy groups on a cdga directly. The homotopy groups of a space will be dual to the homotopy groups of the associated cdga. We will prove that $A$ preserves cofibrations and trivial cofibrations. We only have to check this fact for the generating (trivial) cofibrations in $\sSet$. Note that the contravariance of $A$ means that a (trivial) cofibrations should be sent to a (trivial) fibration. @@ -9,17 +10,25 @@ We will prove that $A$ preserves cofibrations and trivial cofibrations. We only \end{lemma} \begin{proof} Let $\phi \in A(\del \Delta[n])$ be an element of degree $k$, hence it is a map $\del \Delta[n] \to \Apl^k$. We want to extend this to the whole simplex. By the fact that $\Apl^k$ is Kan and contractible we can find a lift $\overline{\phi}$ in the following diagram showing the surjectivity. - - \cimage[scale=0.5]{Extend_Boundary_Form} + \begin{displaymath} + \xymatrix { + \del \Delta[n] \ar[r]^\phi \arcof[d]^i & \Apl^k \\ + \Delta[n] \ar@{-->}[ru]_{\overline{\phi}} + } + \end{displaymath} \end{proof} \begin{lemma} - $A(j) : A(\Delta[n]) \to A(\Lambda^n_k)$ is surjective and a quasi isomorphism. + $A(j) : A(\Delta[n]) \to A(\Lambda^k_n)$ is surjective and a quasi isomorphism. \end{lemma} \begin{proof} As above we get surjectivity from the Kan condition. To prove that $A(j)$ is a quasi isomorphism we pass to the singular cochain complex and use that $C^\ast(j) : C^\ast(\Delta[n]) \we C^\ast(\Lambda^n_k)$ is a quasi isomorphism. Consider the following diagram and conclude that $A(j)$ is surjective and a quasi isomorphism. - - \cimage[scale=0.5]{A_Preserves_WCof} + \begin{displaymath} + \xymatrix { + A(\Delta[n]) \ar[r]^{A(j)} \arwe[d]^\oint & A(\Lambda^k_n) \arwe[d]^\oint \\ + C^\ast(\Delta[n]) \ar[r]^{C^\ast(j)} & C^\ast(\Lambda^k_n) + } + \end{displaymath} \end{proof} Since $A$ is a left adjoint, it preserves all colimits and by functoriality it preserves retracts. From this we can conclude the following corollary. @@ -42,7 +51,7 @@ The induced adjunction in the previous corollary is given by $LA(X) = A(X)$ for } -\section{Homotopy groups of \texorpdfstring{$K(A)$}{K(A)}} +\section{Homotopy groups of cdga's} We are after an equivalence of homotopy categories, so it is natural to ask what the homotopy groups of $K(A)$ are for a cdga $A$. In order to do so, we will define homotopy groups of cdga's directly and compare the two notions. Recall that an augmented cdga is a cdga $A$ with an algebra map $A \tot{\counit} \k$ such that $\counit \unit = \id$. @@ -60,6 +69,7 @@ Note that for a free cdga $\Lambda C$ there is a natural augmentation and the ch Let $A$ be an augmented cdga, then $$ [A, V(n)] \tot{\iso} \Hom_\k(\pi^n(A), \k). $$ } +\todo{prove} We will denote the dual of a vector space as $V^\ast = \Hom_\k(V, \k)$. diff --git a/thesis/notes/Minimal_Models.tex b/thesis/notes/Minimal_Models.tex index 6be3b30..12741de 100644 --- a/thesis/notes/Minimal_Models.tex +++ b/thesis/notes/Minimal_Models.tex @@ -24,10 +24,10 @@ In this section we will discuss the so called minimal models. These are cdga's w $$ (M, d) \we (A, d). $$ \end{definition} -The requirement that there exists a filtration can be replaced by a stronger statement. +In the following lemma we see that the filtration is sometimes naturally there for $1$-reduced cdga's. \begin{lemma} - Let $(A, d)$ be a cdga which is $1$-reduced, quasi-free and with a decomposable differential. Then $(A, d)$ is a minimal algebra. + Let $(A, d)$ be a cdga which is $1$-reduced, such that $A$ is free as cga and $d$ is decomposable. Then $(A, d)$ is a minimal algebra. \end{lemma} \begin{proof} Let $V$ generate $A$. Take $V(n) = \bigoplus_{k=0}^n V^k$ (note that $V^0 = V^1 = 0$). Since $d$ is decomposable we see that for $v \in V^n$: $d(v) = x \cdot y$ for some $x, y \in A$. Assuming $dv$ to be non-zero we can compute the degrees: @@ -35,6 +35,18 @@ The requirement that there exists a filtration can be replaced by a stronger sta As $A$ is $1$-reduced we have $\deg{x}, \deg{y} \geq 2$ and so by the above $\deg{x}, \deg{y} \leq n-1$. Conclude that $d(V(k)) \subset \Lambda(V(n-1))$. \end{proof} +The above definition is the same as in \cite{felix} without assuming connectivity. We find some different definitions of (minimal) Sullivan algebras in the literature. For example we find a definition using well orderings in \cite{hess}. The decomposability of $d$ also admits a different characterization (at least in the connected case). The equivalence of the definitions is expressed in the following two lemmas. + +\Lemma{}{ + A cdga $(\Lambda V, d)$ is a Sullivan algebra if and only if there exists a well order $J$ such that $V$ is generated by $v_j$ for $j \in J$ and $d v_j \in \Lambda V_{ i$. By the above $\oint$ gives a quasi isomorphism on all the terms $sk_i X$. So we are in the following situation: - - \cimage[scale=0.6]{Apl_C_Quasi_Iso_Limit} - - We will define long exact sequences for both sequences in the following way. Consider cochain algebras $B_i$ as follows: - $$ B = \lim_i B_i \dots \fib B_2 \fib^{b_1} B_1 \fib^{b_0} B_0. $$ + \begin{displaymath} + \xymatrix @C=0.3cm{ + A(X) = \lim_i A(sk_i X) \ar[d]^\oint \ar@{-->>}[rr] & & A(sk_2 X) \arfib[r] \arwe[d]^\oint & A(sk_1 X) \arfib[r] \arwe[d]^\oint & A(sk_0 X) \arwe[d]^\oint \\ + C^\ast(X) = \lim_i C^\ast(sk_i X) \ar@{-->>}[rr] & & C^\ast(sk_2 X) \arfib[r] & C^\ast(sk_1 X) \arfib[r] & C^\ast(sk_0 X) + } + \end{displaymath} + + We will define long exact sequences for both sequences in the following way. As the following construction is quite general, consider arbitrary cochain algebras $B_i$ as follows: + \begin{displaymath} + \xymatrix{ + B = \lim_i B_i \ar@{-->>}[rr] & & B_2 \arfib[r]^-{b_1} & B_1 \arfib[r]^-{b_0} & B_0 + } + \end{displaymath} Define a map $t: \prod_i B_i \to \prod_i B_i$ defined by $t(x_0, x_1, \dots) = (x_0 + b_0(x_1), x_1 + b_1(x_2), \dots)$. Note that $t$ is surjective and that $B \iso \ker(t)$. So we get the following natural short exact sequence and its associated natural long exact sequence in homology: $$ 0 \to B \tot{i} \prod_i B_i \tot{t} \prod_i B_i \to 0, $$ - $$ \dots \tot{t_\ast} H^{n-1}(\prod_i B_i) \tot{\Delta} H^n(B) \tot{i_\ast} H^n(\prod_i B_i) \tot{t_\ast} H^n(\prod_i B_i) \tot{\Delta} H^{n+1}(B) \tot{i_\ast} \dots. $$ + $$ \cdots \tot{\Delta} H^n(B) \tot{i_\ast} H^n(\prod_i B_i) \tot{t_\ast} H^n(\prod_i B_i) \tot{\Delta} \cdots $$ In our case we get two such long exact sequences with $\oint$ connecting them. As cohomology commutes with products we get isomorphisms on the left and right in the following diagram. - - \cimage[scale=0.5]{Apl_C_Quasi_Iso_LES} - - So by the five lemma we can conclude that the middle morphism is an isomorphism as well, proving $H^n(A(X)) \tot{\iso} H^n(C^\ast(X))$ for all $n$. This proves the statement for all $X$. + \begin{displaymath} + \xymatrix @C=0.3cm{ + \cdots \ar[r] & H^{n-1}(\prod_i A(sk_i X)) \ar[r] \ariso[d]^\oint & H^n(A(X)) \ar[r] \ar[d]^\oint & H^n(\prod_i A(sk_i X)) \ar[r] \ariso[d]^\oint & \cdots \\ + \cdots \ar[r] & H^{n-1}(\prod_i C^\ast(sk_i X)) \ar[r] & H^n(C^\ast(X)) \ar[r] & H^n(\prod_i C^\ast(sk_i X)) \ar[r] & \cdots \\ + } + \end{displaymath} + + So by the five lemma we can conclude that the middle morphism is an isomorphism as well, proving the isomorphism $H^n(A(X)) \tot{\iso} H^n(C^\ast(X))$ for all $n$. This proves the statement for all $X$. } diff --git a/thesis/notes/Rationalization.tex b/thesis/notes/Rationalization.tex index 994e03d..be74c16 100644 --- a/thesis/notes/Rationalization.tex +++ b/thesis/notes/Rationalization.tex @@ -82,7 +82,7 @@ Having rational cells we wish to replace the cells in a CW complex $X$ by the ra Any simply connected CW complex admits a rationalization. } \Proof{ - Let $X$ be a CW complex. We will define $X_\Q$ with induction on the dimension of the cells. Since $X$ is simply connected we can start with $X^0_\Q = X^1_\Q = \ast$. Now assume that the rationalization $X^k \tot{\phi^k} X^k_\Q$ is already defined. Let $A$ be the set of $k+1$-cells and $f_\alpha : S^k \to X^{k+1}$ be the attaching maps. Then by \LemmaRef{SnQ-extension} these extend to $g_\alpha = (\phi^k \circ f_\alpha)' : S^k_\Q \to X^k_\Q$. This defines $X^{k+1}_\Q$ as the pushout in the following diagram. + Let $X$ be a CW complex. We will define $X_\Q$ with induction on the dimension of the cells. Since $X$ is simply connected we can start with $X^0_\Q = X^1_\Q = \ast$. Now assume that the rationalization $X^k \tot{\phi^k} X^k_\Q$ is already defined. Let $A$ be the set of $k+1$-cells and $f_\alpha : S^k \to X^{k+1}$ be the attaching maps. Then by \LemmaRef{SnQ-extension} these extend to $g_\alpha = (\phi^k \circ f_\alpha)' : S^k_\Q \to X^k_\Q$. This defines $X^{k+1}_\Q$ as the pullback in the following diagram. \begin{displaymath} \xymatrix{ diff --git a/thesis/notes/Serre.tex b/thesis/notes/Serre.tex index 68306a1..a129587 100644 --- a/thesis/notes/Serre.tex +++ b/thesis/notes/Serre.tex @@ -85,6 +85,7 @@ In the following arguments we will consider fibrations and need to compute homol &\iso (H_\ast(B^{k+1}, B^k) \tensor H_\ast(F)) \\ &= \bigoplus_{j+l=i+1} H_j(B^{k+1}, B^k) \tensor H_l(F) \end{align*} + \todo{Write down actual kunneth theorem with torsion (still works though)} Note that this is the graded tensor product, and that the term $H_{i+1}(B^{k+1}, B^k) \tensor H_0(F) = H_{i+1}(B^{k+1}, B^k)$ and that this identification is compatible with the induced map $p_\ast : H_{i+1}(E^{k+1}, E^k) \to H_{i+1}(B^{k+1}, B^k)$ (hence the map is surjective). To prove that the map is a $\C$-iso, we need to prove that the kernel is in $\C$. The kernel is the sum of the following terms, with $1 \leq l \leq i+1$: $$ H_{i+1-l}(B^{k+1}, B^k) \tensor H_l(F). $$ Now we can use the assumption that $H_l(F) \in \C$ for $1 \leq l < n$ and that for $B \in \C$ we have $A \tensor B \in \C$ for all $A$ (by \LemmaRef{Serre-properties}). This concludes that the kernel $H_{i+1-l}(B^{k+1}, B^k) \tensor H_l(F)$ is indeed in $\C$. And hence the induced map is a $\C$-iso for all diff --git a/thesis/preamble.tex b/thesis/preamble.tex index 4ce3665..ebbb519 100644 --- a/thesis/preamble.tex +++ b/thesis/preamble.tex @@ -76,6 +76,7 @@ \newcommand{\arfib}{\ar@{->>}} \newcommand{\artfib}{\ar@{->>}|\simeq} \newcommand{\arwe}{\ar|-\simeq} +\newcommand{\ariso}{\ar|-\iso} % adjunction symbol for xymatrices \newcommand{\xyadj}{\raisebox{0.2\height}{\scalebox{0.5}{$\perp$}}}