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A299402 Lexicographic first sequence of positive integers such that a(n)*a(n+1) has a digit 2, and no term occurs twice. 14

%I

%S 1,2,6,4,3,7,16,8,9,14,13,17,12,10,20,11,19,15,18,24,5,25,21,22,26,27,

%T 23,34,28,29,32,31,33,37,35,36,42,30,40,38,39,48,43,44,46,45,47,41,49,

%U 50,51,52,53,54,55,59,58,56,57,60,62,61,66,64,63,67,69,68,65,80,74,71,72,73,77,76,70,75,79,78,84,83,81,82,86,87,95,96,92

%N Lexicographic first sequence of positive integers such that a(n)*a(n+1) has a digit 2, and no term occurs twice.

%C A permutation of the positive integers.

%e a(1) = 1 is the least positive integer, it has no other requirement to satisfy.

%e a(2) = 2 is the least positive integer > a(1) = 1, and a(2)*a(1) = 2 has a digit 2.

%e a(3) = 6 is the least positive integer > a(2) = 2 such that a(3)*a(2) (= 12) has a digit 2: The smaller choices 3, 4 or 5 do not satisfy this.

%e a(4) = 4 is the least positive integer > a(2) = 2 such that a(4)*a(3) (= 24) has a digit 2: The smaller choice 3 yields 3*6 = 18 and does not satisfy this.

%e Now, the least available positive integer a(5) = 3 is such that 3*4 = 12, which has again a digit 2. And so on.

%o (PARI) a(n,f=1,d=2,a=1,u=[a])={for(n=2,n,f&&if(f==1,print1(a","),write(f,n-1," "a));for(k=u[1]+1,oo,setsearch(u,k)&&next;setsearch(Set(digits(a*k)),d)&&(a=k)&&break);u=setunion(u,[a]);while(#u>1&&u[2]==u[1]+1,u=u[^1]));a}

%Y Cf. A299403, A298974, ..., A298979 (analog with digit 3, ..., 9).

%Y Cf. A299957, A299969, ..., A299988 (analog with addition instead of multiplication, and different digits).

%K nonn,base

%O 1,2

%A _M. F. Hasler_, Feb 22 2018

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Last modified January 20 08:07 EST 2020. Contains 331081 sequences. (Running on oeis4.)