login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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
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
Sequence in context: A035768 A107564 A135648 * A255867 A326018 A202051
KEYWORD
nonn
AUTHOR
Massimo Kofler, Oct 18 2023
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

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