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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 (list; graph; refs; listen; history; text; internal format)
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

Harvey P. Dale, Table of n, a(n) for n = 1..1000

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 *)

CROSSREFS

Cf. A001358, A001750, A050674-A050719, A050805, A050806.

Sequence in context: A171133 A152246 A069823 * A031888 A299469 A046291

Adjacent sequences:  A133318 A133319 A133320 * A133322 A133323 A133324

KEYWORD

base,easy,nonn

AUTHOR

Jonathan Vos Post, Oct 18 2007

EXTENSIONS

More terms from R. J. Mathar, Oct 22 2007

STATUS

approved

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Last modified September 20 14:44 EDT 2021. Contains 347586 sequences. (Running on oeis4.)