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A256935 Concatenation of odd prime factors of numbers whose digits are all odd. 1

%I #17 Apr 14 2015 16:45:29

%S 0,3,5,7,33,11,13,35,17,19,31,311,57,37,313,317,53,511,319,59,71,73,

%T 355,711,79,713,331,519,97,3311,337,113,523,3313,717,131,719,3335,137,

%U 139,151,3317,531,157,353,3319,173,557,359,179,191,193,3513,197,199,311,313,3357,317,1129,331,3337,567

%N Concatenation of odd prime factors of numbers whose digits are all odd.

%C In decimal digits of a(n) there is at least one prime.

%e a(5) = 33 because the fifth odd number is 9, and the odd prime factors of 9 are 3 * 3, thus 33 is the result of the concatenation of these factors.

%t f[n_] := Block[{of = Select[Table[#1, {#2}] & @@@ FactorInteger@ n // Flatten, PrimeQ@ # && # > 2 &]}, IntegerDigits@ of // Flatten // FromDigits]; f /@ Select[Range@ 360, OddQ[Times @@ IntegerDigits[#]] &] (* _Michael De Vlieger_, Apr 13 2015 *)

%Y Cf. A119603, A256154 (Concatenation of odd prime factors of m such that the decimal digits of m only have odd prime factors).

%K base,nonn

%O 1,2

%A _Giovanni Teofilatto_, Apr 13 2015

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