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 A358544 a(n) is the smallest number with exactly n divisors that are centered triangular numbers. 4
 1, 4, 20, 320, 460, 5440, 14260, 12920, 168640, 103360, 594320, 3878720, 2377280, 9211960, 18423920, 36847840, 125995840, 73695680, 865924240, 976467760, 1952935520, 3463696960, 3905871040, 31246968320, 22946992360 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Any subsequent terms are > 10^10. - Lucas A. Brown, Dec 24 2022 LINKS Table of n, a(n) for n=1..25. Lucas A. Brown, Python program. Eric Weisstein's World of Mathematics, Centered Triangular Number Index entries for sequences related to divisors of numbers EXAMPLE a(3) = 20 because 20 has 3 centered triangular divisors {1, 4, 10} and this is the smallest such number. PROG (PARI) isct(n) = my(k=(2*n-2)/3, m); (n==1) || ((denominator(k)==1) && (m=sqrtint(k)) && (m*(m+1)==k)); \\ A005448 a(n) = my(k=1); while (sumdiv(k, d, isct(d)) != n, k++); k; \\ Michel Marcus, Nov 21 2022 CROSSREFS Cf. A005179, A005448, A130317, A300409, A358542, A358545. Sequence in context: A120599 A012797 A342907 * A167002 A227005 A054465 Adjacent sequences: A358541 A358542 A358543 * A358545 A358546 A358547 KEYWORD nonn,more AUTHOR Ilya Gutkovskiy, Nov 21 2022 EXTENSIONS a(14) from Michel Marcus, Nov 21 2022 a(15)-a(25) from Jinyuan Wang, Nov 29 2022 STATUS approved

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Last modified September 13 07:44 EDT 2024. Contains 375880 sequences. (Running on oeis4.)