OFFSET
1,1
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
KEYWORD
nonn
AUTHOR
Massimo Kofler, Oct 18 2023
STATUS
approved