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 A363821 Lesser of 2 successive squarefree semiprimes (k, k+4) sandwiching 3 consecutive nonsquarefree numbers. 2
 1923, 2573, 2823, 3427, 3574, 3869, 3949, 4474, 4849, 4911, 5191, 5489, 5633, 5947, 6927, 7153, 7323, 7397, 7405, 7441, 7747, 7985, 8223, 8347, 8403, 8917, 9249, 9385, 9557, 9773, 10042, 10447, 10673, 10681, 11659, 12219, 13349, 13423, 13849, 14209, 14273, 16323, 16973 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Table of n, a(n) for n=1..43. EXAMPLE 1923 = 3 * 641 and 1927 = 41 * 47 are semiprime numbers; 1924 = 2^2 * 13 * 37, 1925 = 5^2 * 7 * 11 and 1926 = 2 * 3^2 * 107 are 3 consecutive nonsquarefree numbers, so 1923 is a term. 2573 = 31 * 83 and 2577 = 3 * 859 are semiprime numbers; 2574 = 2 * 3^2 * 11 * 13, 2575 = 5^2 * 103 and 2576 = 2^4 * 7 * 23 are 3 consecutive nonsquarefree numbers, so 2573 is a term. MATHEMATICA Select[Partition[Select[Range[17000], SquareFreeQ], 2, 1], Differences[#] == {4} && PrimeOmega[#] == {2, 2} &][[;; , 1]] (* Amiram Eldar, Oct 18 2023 *) PROG (PARI) is1(n) = (bigomega(n)==2) && (omega(n)==2); \\ A006881 is9(n) = !issquarefree(n); \\ A013929 isok(k) = is1(k) && is9(k+1) && is9(k+2) && is9(k+3) && is1(k+4); \\ Michel Marcus, Oct 22 2023 CROSSREFS Cf. A006881, A013929. Sequence in context: A035768 A107564 A135648 * A255867 A326018 A202051 Adjacent sequences: A363818 A363819 A363820 * A363822 A363823 A363824 KEYWORD nonn AUTHOR Massimo Kofler, Oct 18 2023 STATUS approved

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Last modified July 13 08:08 EDT 2024. Contains 374274 sequences. (Running on oeis4.)