cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A362003 Squarefree composite numbers m such that k - m^2 < m, where k is the smallest number greater than m^2 such that rad(k) | m.

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%I A362003 #30 Apr 23 2023 22:43:52
%S A362003 42,66,78,362,1086,1122,1254,1794,1810,1846,1974,2534,2730,3318,3982,
%T A362003 4890,5538,5590,6006,6214,9230,12922,12990,13515,15510,16205,17430,
%U A362003 18642,20306,22170,23170,25098,26962,27030,29274,31070,32142,32410
%N A362003 Squarefree composite numbers m such that k - m^2 < m, where k is the smallest number greater than m^2 such that rad(k) | m.
%C A362003 Most small squarefree m have k - m^2 > m. For prime m = p, k = p^3, hence (p^3 - p^2) > p.
%F A362003 This sequence is { m : A362045(n) - m^2 < m and m in A120944 }.
%e A362003 a(1) = 42 since 42 is the smallest squarefree number such that the smallest k > m^2 such that rad(k) | m also has difference k - m^2 < m.
%e A362003 Table showing a(n) = A120944(i) = m, A362045(i) = k, and the difference k-m^2.
%e A362003     i      m         k  (k-m^2)
%e A362003   -----------------------------
%e A362003    14     42      1792      28
%e A362003    22     66      4374      18
%e A362003    27     78      6144      60
%e A362003   147    362    131072      28
%e A362003   478   1086   1179648     252
%e A362003   495   1122   1259712     828
%e A362003   558   1254   1572864     348
%e A362003   813   1794   3219264     828
%e A362003   822   1810   3276800     700
%e A362003   840   1846   3407872     156
%e A362003   900   1974   3898368    1692
%t A362003 s = Select[Range[6, 400], And[CompositeQ[#], SquareFreeQ[#]] &]; Reap[Do[(m = #^2 + 1; While[! Divisible[#, Times @@ FactorInteger[m][[All, 1]]], m++]; If[m - #^2 < #, Sow[#]]) &[s[[i]]], {i, Length[s]}] ][[-1, -1]]
%o A362003 (PARI) rad(n) = factorback(factorint(n)[, 1]); \\ A007947
%o A362003 isok(m) = if (!isprime(m) && issquarefree(m), for (k=1+m^2, m+m^2, if (!(m % rad(k)), return(1)))); \\ _Michel Marcus_, Apr 21 2023
%Y A362003 Cf. A007947, A120944, A362045.
%K A362003 nonn
%O A362003 1,1
%A A362003 _Michael De Vlieger_, Apr 05 2023