Unique-neighbor expander

I want to solve Problem 4.10 from Randomness by Salil Vadhan. https://people.seas.harvard.edu/~salil/cs225/spring15/PS3.pdf

Consider a bipartite expander $ G$ with left degree $ D$ so that every subset $ S$ of the left vertices with at most $ K$ vertices has at least $ (1-\epsilon)D|S|$ neighbors. Then $ G$ also has the property that it has $ (1-2\epsilon)D|S|$ unique neighbors. Unique meaning that it has exactly one corresponding vertex from $ S$ .

I recognize that the new expansion factor is $ (1-2\epsilon)D = 2\cdot(1-\epsilon)D -D$

Understanding Gillman’s proof of the Chernoff bound for expander graphs

My question is about Claim 1 in the proof here: Gillman (1993). At the end of the proof, the author says:

The matrix product $ U^\top\sqrt{D^{-1}}(P+(\mathrm{e}^x-1)B(0)-\mu I)\sqrt{D}U$ , which is equal to $ (D’-\mu I)(I+(D’-\mu I)^{-1}(\mathrm{e}^x-1)D’U^\top D_A U)$ , is singular. Therefore,

\begin{align*} 1&\leq \lVert (D’-\mu I)^{-1}(\mathrm{e}^x-1)D’U^\top D_A U \rVert_2 \ &\leq \frac{1}{\mu-\lambda_2}(\mathrm{e}^x -1). \end{align*}

(The first inequality uses the continuity of the function $ \lambda_2(y)$ .)

I understand why the two expressions at the beginning are equal and I understand the second inequality, but I do not understand the first inequality and also why the matrix product is singular.

Let me provide the definitions so you can avoid reading the whole paper. There is a weighted undirected graph $ G=(V, E, w)$ where $ w_{ij}=0$ if $ \{i,j\}\notin E$ . Denote $ w_i:=\sum_j w_{ij}$ . Let $ P$ denote the transition matrix, so $ P_{ij}:=\frac{w_{ij}}{w_i}$ . Denote by $ \lambda_2$ the second largest eigenvalue of $ P$ and by $ \epsilon:=1-\lambda_2$ the spectral gap. Next, let $ M$ be the weighted adjacency matrix $ M_{ij}:=w_{ij}$ . Let $ A$ be a set of vertices and $ \chi_A$ be an indicator function. Some more definitions are:

\begin{align*} &E_r:=\operatorname{diag}(\mathrm{e}^{r\chi_A}) & &P(r):=PE_r \ &D:=\operatorname{diag}(\frac{1}{w_i}) & &S:=\sqrt{D}M\sqrt{D} \ &S_r:=\sqrt{DE_r}M\sqrt{DE_r} & & B(r):=\frac{1}{\mathrm{e}-1}(P(r+1)-P(r)) \end{align*}

Moreover, since $ S$ is unitarily diagonalizable, there exist a unitary matrix $ U$ and a diagonal matrix $ D’$ such that $ D’=U^\top SU$ . Furthermore, there exists a diagonal matrix $ D_A$ such that $ B(0)=PD_A$ .

Define $ \lambda(r)$ to be the largest eigenvalue of $ P(r)$ and $ \lambda_2(r)$ to be its second largest eigenvalue. As before, $ \epsilon_r := \lambda(r)-\lambda_2(r)$ is the spectral gap.

In Claim 1 the author lets $ 0\leq x\leq r$ be some number. He also defines $ \mu<\lambda(x)$ to be any other eigenvalue of $ P(x)$ . At the end of the proof, we are only interested in $ \mu>\lambda_2$ .

Some other facts are:

\begin{align*} &\lVert D’ \rVert_2 = \lVert D_A \rVert_2 = 1 & &D’=U^\top\sqrt{D^{-1}}P\sqrt{D}U \ &P(0)=P & &\lambda(0)=1 & &\lambda_2(0)=\lambda_2 \ &P=\sqrt{D}S\sqrt{D^{-1}} & &P(r)=\sqrt{DE_r^{-1}}S_r\sqrt{E_rD^{-1}} \end{align*}

I hope I didn’t miss anything relevant. Thank you for your help.

Unit testing extension method adding view location expander


I’m writing unit tests for an extension method I wrote. Its only purpose it is to wrap startup logic which extends ViewLocationExpanders list by an instance implementating IViewLocationExpander. ViewLocationExpanders is a property of RazorViewEngineOptions, which can be configured in application startup in ConfigureServices() method. I’m using XUnit 2.4.1.


Instead of:

services.Configure<RazorViewEngineOptions>(options => {     options.ViewLocationExpanders.Add(new ViewLocationExpander()); }); 

I can use:

services.AddViewLocationExpander(new ViewLocationExpander()); 


public class ViewLocationExpander : IViewLocationExpander {     public IEnumerable<string> ExpandViewLocations(         ViewLocationExpanderContext context,          IEnumerable<string> viewLocations)     {         if (context == null)         {             throw new ArgumentNullException(nameof(context));         }          if (viewLocations == null)         {             throw new ArgumentNullException(nameof(viewLocations));         }          /*             * Note:             * {0} = action name             * {1} = controller name             * {2} = area name             */          var newViewLocations = new string[]         {             // Example: '/Views/Home/_Partials/FooBar.cshtml'             "/Views/{1}/_Partials/{0}.cshtml",         };          // Add new locations *AFTER* MVC default locations.         return viewLocations.Union(newViewLocations);     }      public void PopulateValues(ViewLocationExpanderContext context)     {         context.Values["customviewlocation"] = nameof(ViewLocationExpander);     } } 

Extension method

public static IServiceCollection AddViewLocationExpander(     this IServiceCollection services,      IViewLocationExpander viewLocationExpander) {     if (services == null)     {         throw new ArgumentNullException(nameof(services));     }      if (viewLocationExpander == null)     {         throw new ArgumentNullException(nameof(viewLocationExpander));     }      return services.Configure<RazorViewEngineOptions>(options =>     {         options.ViewLocationExpanders.Add(viewLocationExpander);     }); } 

Unit tests

[Fact] public void ExtensionMethodAddsNewViewLocationExpander() {     // Arrange     var services = new ServiceCollection();     services.AddMvc();      // These two are required to active the RazorViewEngineOptions.     services.AddSingleton<IHostingEnvironment, HostingEnvironment>();     services.AddSingleton<ILoggerFactory, LoggerFactory>();      // Act     var serviceProvider = services.BuildServiceProvider();     var oldOptions = serviceProvider.GetRequiredService<IOptions<RazorViewEngineOptions>>().Value;     services.AddViewLocationExpander(new ViewLocationExpander());     serviceProvider = services.BuildServiceProvider();     var newOptions = serviceProvider.GetRequiredService<IOptions<RazorViewEngineOptions>>().Value;      // Assert     Assert.True(newOptions.ViewLocationExpanders.Count > oldOptions.ViewLocationExpanders.Count); } 


  1. Am I going beyond the scope of what unit testing should include? I’m afraid that my code is actually testing the basic functions of generic collections and/or aspects of ASP.NET Core.
  2. If my concern above is true, that should I write unit tests to this at all? How should it work?

Ambiguous chevron styles in an expander

I’ve begun to notice that a lot of apps have opted to use a right arrow chevron on the right edge of the header.

However, I am used to seeing it to indicate that it will transition to a different page. This is common in mobile applications as well as some desktop applications like firefox overflow menu’s.

From my personal observation and experience, a right arrow chevron, if used as an expander, should not be on the right side. It can be either on the left of the header or a different icon like a down arrow chevron.

For example, I find the following pictures quite misleading, and I believe in their current positions, they should be down arrow chevrons instead.

Am I correct to say this? I realize there isn’t a black or white solution, but I’m leaning towards my reasoning. But because a lot of apps made specifically for UI designs do in fact follow this pattern makes me uncertain.

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