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 A045882 Smallest term of first run of (at least) n consecutive integers which are not squarefree. 26
 4, 8, 48, 242, 844, 22020, 217070, 1092747, 8870024, 221167422, 221167422, 47255689915, 82462576220, 1043460553364, 79180770078548, 3215226335143218, 23742453640900972, 125781000834058568 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Solution for n=10 is same as for n=11. This sequence is infinite and each term initiates a suitable arithmetic progression with large differences like squares of primorials or other suitable products of primes from prime factors being on power 2 in terms and in chains after. Proof includes solution of linear Diophantine equations and math. induction. See also A068781, A070258, A070284, A078144, A049535, A077640, A077647, A078143 of which first terms are recollected here. - Labos Elemer, Nov 25 2002 REFERENCES J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 242, p. 67, Ellipses, Paris 2008. LINKS L. Marmet, First occurrences of square-free gaps and an algorithm for their computation, arXiv preprint arXiv:1210.3829 [math.NT], 2012. See also the author page. Eric Weisstein's World of Mathematics, Squarefree numbers Eric Weisstein's World of Mathematics, Squareful FORMULA a(n) = 1 + A020754(n+1). - R. J. Mathar, Jun 25 2010 EXAMPLE a(3) = 48 as 48, 49 and 50 are divisible by squares. n=5 -> {844=2^2*211; 845=5*13^2; 846=2*3^2*47; 847=7*11^2; 848=2^4*53}. MATHEMATICA cnt = 0; k = 0; Table[While[cnt < n, k++; If[! SquareFreeQ[k], cnt++, cnt = 0]]; k - n + 1, {n, 7}] PROG (PARI) a(n)=my(s); for(k=1, 9^99, if(issquarefree(k), s=0, if(s++==n, return(k-n+1)))) \\ Charles R Greathouse IV, May 29 2013 CROSSREFS Cf. A013929, A053806, A049535, A077647, A078143. Also A069021 and A051681 are different versions. Sequence in context: A078236 A054881 A076687 * A051681 A267987 A056407 Adjacent sequences:  A045879 A045880 A045881 * A045883 A045884 A045885 KEYWORD nonn,nice,hard,changed AUTHOR EXTENSIONS a(9)-a(11) from Patrick De Geest, Nov 15 1998, Jan 15 1999 a(12)-a(15) from Louis Marmet (louis(AT)marmet.org) and David Bernier (ezcos(AT)yahoo.com), Nov 15 1999 a(16) was obtained as a result of a team effort by Z. McGregor-Dorsey et al. [Louis Marmet (louis(AT)marmet.org), Jul 27 2000] a(17) was obtained as a result of a team effort by E. Wong et al. [Louis Marmet (louis(AT)marmet.org), Jul 13 2001] a(18) = 125781000834058568 was obtained as a result of a team effort by L. Marmet et al. STATUS approved

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Last modified October 17 04:09 EDT 2019. Contains 328106 sequences. (Running on oeis4.)