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 A209799 Composite numbers n such that the concatenation of the digits of the prime divisors of n is a prime number. 3
 4, 6, 8, 9, 12, 16, 18, 21, 22, 24, 25, 27, 32, 33, 36, 39, 44, 46, 48, 49, 51, 54, 58, 63, 64, 66, 70, 72, 81, 82, 88, 92, 93, 96, 99, 108, 111, 115, 116, 117, 121, 125, 128, 132, 133, 140, 141, 142, 144, 147, 153, 154, 159, 162, 164, 165, 166, 169, 176, 177 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Table of n, a(n) for n=1..60. EXAMPLE 70 is in the sequence because the prime divisors of 70 are {2,5,7} and 257 is prime. MAPLE read("transforms") ; isA209799 := proc(n) local pdivs ; if isprime(n) or n < 4 then return false; end if; pdivs := sort(convert(numtheory[factorset](n), list)) ; isprime(digcatL(pdivs)) ; end proc: for n from 4 to 200 do if isA209799(n) then printf("%d, ", n) ; end if; end do: # R. J. Mathar, Mar 19 2012 MATHEMATICA Select[Range[200], CompositeQ[#]&&PrimeQ[FromDigits[Flatten[ IntegerDigits/@ FactorInteger[#] [[;; , 1]]]]]&] (* Harvey P. Dale, Apr 10 2023 *) CROSSREFS Cf. A000040, A084317, A084318. Sequence in context: A079142 A062002 A090419 * A028958 A200878 A344030 Adjacent sequences: A209796 A209797 A209798 * A209800 A209801 A209802 KEYWORD nonn,base,less AUTHOR Michel Lagneau, Mar 13 2012 STATUS approved

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Last modified April 16 22:53 EDT 2024. Contains 371755 sequences. (Running on oeis4.)