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Bir Kafle, Florian Luca , and Alain Togbé, <a href="https://doi.org/10.33039/ami.2020.09.002">Pentagonal and heptagonal repdigits</a>, Annales Mathematicae et Informaticae, pp. 137-145.
Bir Kafle, Florian Luca and Alain Togbé, <a href="https://doi.org/10.33039/ami.2020.09.002">Pentagonal and heptagonal repdigits</a>, Annales Mathematicae et Informaticae. , pp. 137-145.
INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=341">Encyclopedia of Combinatorial Structures 341</a>.
Leo Tavares, <a href="/A000566/a000566.jpg">Illustration</a>.
Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/HeptagonalNumber.html">Heptagonal Number</a>.
<a href="/index/Pol#polygonal_numbers">Index to sequences related to polygonal numbers</a>.
David Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, Revised edition 1987. See p. 123.
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Comment from Ken Rosenbaum, Dec 02 2009: if you multiply the terms of this sequence by 40 and add 9, you get A017354, which is the list of squares of all whole numbers ending in 7 (this is easy to prove).
40*a(n)+ 9 = A017354(n-1). - Ken Rosenbaum, Dec 02 2009.
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