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Testing pentagonal numbers

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The nth pentagonal number Pn follows the formula Pn = (3n² − n)/2 for positive integer n. For non-positive integer n, the same formula defines a generalized pentagonal number.

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The nth pentagonal number Pn follows the formula Pn = (3n² − n)/2 for positive integer n. For non-positive integer n, the same formula defines a generalized pentagonal number.

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hn17johndcook-com4
01

Three Examples Suffice

The blog post demonstrates that three carefully chosen examples can be sufficient to illustrate a mathematical or computational principle. It explores the idea that minimal examples help clarify core concepts without unnecessary complexity, supporting efficient learning and communication.

hnscience
2.0
02

Partial fraction decomposition

Partial fraction decomposition is commonly introduced in calculus as a technique for integrating rational functions by breaking P(x)/Q(x) into simpler terms. However, the post suggests that this method has applications beyond integration that are often overlooked in a typical calculus class.

johndcook-comscience
0.3
03

Identifier length and limited programmer memory (2009)

A 2009 study finds that longer identifiers in source code, while more descriptive, increase cognitive load on programmers' working memory, potentially reducing code comprehension. The research suggests an optimal identifier length that balances descriptiveness with memory constraints for better software readability and maintenance.

hnscience
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  • The article describes a compact 26-page Fortran implementation that performs full 3D optimization for Mars missions, highlighting the efficiency of legacy coding approaches for complex space trajectory calculations.

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  • The article argues against viewing the human brain as a computational machine, emphasizing that biological cognition differs fundamentally from artificial intelligence in its embodied, emotional, and social dimensions.

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  • The article explores a geometric problem: given N vectors in a d-dimensional space, can we always find a vector that is not orthogonal to any of them? It presents the answer depends on N and d, connecting the problem to concepts like measure theory, the Borsuk-Ulam theorem, and the ham sandwich theorem, ultimately showing a threshold exists based on the dimension.

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  • A blog post discusses a mathematical identity where pentagonal numbers can be expressed in terms of triangular numbers. It highlights that while examples don't typically prove theorems, in this case the identity Pn = T(2n−1) − T(n−1) holds, showing that three examples can suffice for proving certain relationships.

    johndcook-com#Science
  • A Math StackExchange discussion explains that square matrices are necessary for operations like inversion, determinant, and eigenvalues, which are undefined for non-square matrices. Only square matrices can have both left and right inverses, making them key for unique-solution linear systems and transformations from a space to itself.

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  • The nth pentagonal number Pn follows the formula Pn = (3n² − n)/2 for positive integer n. For non-positive integer n, the same formula defines a generalized pentagonal number.

    johndcook-com#Science

  • John D. Cook describes how a sequence of his blog posts often follows a hidden thread, beginning with a post about the mathematical approximation exp(−x²) ≈ (1 + cos(sin(x) + x))/2, which some commenters incorrectly attributed solely to a first-order Taylor expansion.

    johndcook-com#Science