COSESAI

Central Limit Theorem Demonstrator cover — Watch the bell curve emerge from any source distribution

Watch the Bell Curve Emerge from Anything: Inside the Central Limit Theorem Demonstrator

By xiezixing 2026年7月6日

An interactive visual proof that the distribution of sample means converges to a Normal bell curve regardless of source distribution, plus four controls and four source distributions you can dial in.

JSON-LD Generator from CSV cover showing schema.org JSON-LD output and three validator decisions

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Three Spreadsheet Decisions That Decide Whether JSON-LD Survives the Validator

By xiezixing 2026年7月6日

How the schema.org type, public page URL, and spreadsheet column names decide whether the output JSON-LD earns a Google rich result.

PKCE Code Verifier and Challenge Generator — Three modes that turn invalid_grant into a concrete check

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Three PKCE Modes That Turn invalid_grant Into a Concrete Check

By xiezixing 2026年7月5日

Generate, validate, and verify OAuth 2.0 PKCE code_verifier and code_challenge pairs against RFC 7636 — without round-tripping through the identity provider.

Cover for Physics Kinematics Problem Solver article showing SUVAT equations

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The Kinematics Toolkit Most Students Only Learn Half Of

By xiezixing 2026年7月5日

Walk through SUVAT equations and projectile motion end-to-end, then see how the Physics Kinematics Problem Solver walks the algebra step by step.

PDF to Markdown Converter — convert PDF documents to Markdown with structure preserved

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PDF to Markdown: How to Get Publish-Ready Markdown From Any PDF

By xiezixing 2026年7月5日

A free browser-based PDF to Markdown converter that runs entirely client-side — extract headings, lists, tables, and code blocks from any PDF without uploading a single byte to a server.

Credit Card Validator cover image — why Luhn is a typo catcher not a security check, by Elysia Tools blog

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Why the Luhn Check Is a Typo Catcher, Not a Security Check: Inside a Modern Credit Card Validator

By xiezixing 2026年7月5日

The four-part output every payment form's credit card validator should surface — Luhn, brand, length, IIN — and the prefix-table traps that old validators miss.

JSON to CSV Converter: three decisions for reliable exports — featured poster

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Three choices that decide whether your JSON→CSV export survives the next person who opens it

By xiezixing 2026年7月5日

JSON to CSV is one click — but it hides three decisions that decide whether the export survives: delimiter, nested-object handling, and date format.

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The Three Page-Range Choices That Decide Whether Your PDF Text Is Actually Useful

By xiezixing 2026年7月5日

PDF Text Extractor makes three decisions explicit — which pages, what output format, and whether to preserve the layout — and those three choices are the difference between clean text and paragraph-spaghetti.

Amortization Schedule Three Numbers That Decide Your Loan cover

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The Three Numbers That Quietly Decide Your Loan

By xiezixing 2026年7月4日

Principal, rate, and term give you the headline payment. The real cost of a loan hides in three downstream decisions: frequency, extra principal, and the payoff-vs-investment trade-off.