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 A309556 Composite numbers m such that m divides Sum_{k=1..m-1} (lcm(1,2,...,(m-1)) / k)^2. 0
 52781, 782957, 1395353, 2602439 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Composites m such that m | H_2(m-1) * lcm(1^2,2^2,...,(m-1)^2), where H_2(m) = 1/1^2 + 1/2^2 + ... + 1/m^2. By Wolstenholme's theorem, if p > 3 is a prime, then p divides the numerator of H_2(p-1) and thus H_2(p-1) * lcm(1,2^2,...,(p-1)^2) == 0 (mod p). This sequence is formed by the pseudoprimes that are solutions of this congruence. a(5) > 10^7 if it exists. LINKS Eric Weisstein's World of Mathematics, Wolstenholme's Theorem. Wikipedia, Wolstenholme's theorem. MATHEMATICA s = 0; c = 1; n = 1; seq = {}; Do[s += 1/n^2; c = LCM[c, n^2]; n++; If[CompositeQ[n] && Divisible[s*c, n], AppendTo[seq, n]], {2 * 10^6}]; seq CROSSREFS Cf. A007406, A007407, A025529 (see our comment), A051418. Sequence in context: A185530 A057851 A058326 * A206092 A233965 A157758 Adjacent sequences:  A309553 A309554 A309555 * A309557 A309558 A309559 KEYWORD nonn,bref,more AUTHOR Amiram Eldar and Thomas Ordowski, Aug 07 2019 STATUS approved

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Last modified March 31 03:08 EDT 2020. Contains 333136 sequences. (Running on oeis4.)