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 Dec17 awarded Caucus Dec14 awarded Yearling Oct21 awarded Notable Question Sep24 awarded Autobiographer Jul2 awarded Curious Apr5 awarded Popular Question Jan24 awarded Popular Question May6 revised Is $R_K$ first countable? Edited title. May6 suggested approved edit on Is $R_K$ first countable? Apr30 accepted Proof by Induction solution not understood Apr30 comment Proof by Induction solution not understood Thanks. What about this part? $(3m + 1)(3m + 2)(3m + 3)≤(3m + 3)^3 = 27(m + 1)^3$ Apr30 awarded Citizen Patrol Apr30 asked Proof by Induction solution not understood Apr26 awarded Organizer Apr26 revised Orthogonality & Adjoint Operator Added homework tag Apr26 suggested approved edit on Orthogonality & Adjoint Operator Apr22 comment Injection / Surjection I totally get that now. Thanks! Apr22 comment Injection / Surjection @vadim123 Well yes, but I don't know if it's injective or not at the start. The goal is to find out. So 4 & -4 are both elements of A, right? And they can't both be mapped to the same element in B, making the function NOT injective, right? Apr22 comment Injection / Surjection @TheChaz2.0 This is how I like to define it. Injective means that every member of "A" has its own unique matching member in "B".link Apr22 comment Injection / Surjection Sorry, I'm just trying to get my head around this. So if $x = 1$, it's compliment is $-1$, right? But for injection, $x$ must equal $x'$. Can you give me an example where $x$ actually equals to $x'$. I can't think of any.