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A133321
Inserting any (identical) digit between adjacent digits of an odd semiprime k never yields a prime.
1
15, 25, 35, 55, 65, 85, 95, 115, 121, 143, 145, 155, 185, 187, 205, 215, 235, 253, 265, 295, 299, 305, 335, 341, 355, 365, 393, 395, 411, 415, 437, 445, 451, 473, 485, 505, 515, 535, 545, 565, 583, 635, 655, 671, 679, 685, 695, 717, 745, 755, 781, 785, 815
OFFSET
1,1
COMMENTS
Odd semiprime analog of A050805. Trivially true for any digit if we substitute "even semiprime" for "odd semiprime." Trivially true for any semiprime which is a multiple of 5 (A001750). The nonmultiples of 5 in this sequence begin 121, 143, 187, 253, 299, 341.
LINKS
EXAMPLE
121 is in the sequence because 10201, 11211, 12221, 13231, 14241, 15251, 16261, 17271, 18281, 19291 are all composite.
MATHEMATICA
Select[Select[Range[11, 900, 2], PrimeOmega[#]==2&], AllTrue[Table[ FromDigits[ Riffle[ IntegerDigits[#], n]], {n, 0, 9}], CompositeQ]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 07 2018 *)
KEYWORD
base,easy,nonn
AUTHOR
Jonathan Vos Post, Oct 18 2007
EXTENSIONS
More terms from R. J. Mathar, Oct 22 2007
STATUS
approved