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A298979 Lexicographic first sequence of positive integers such that a(n)*a(n+1) has a digit 9, and no term occurs twice. 15
1, 9, 10, 19, 5, 18, 11, 27, 7, 13, 3, 23, 4, 24, 8, 12, 16, 6, 15, 26, 35, 14, 21, 29, 17, 37, 25, 36, 22, 41, 34, 28, 32, 30, 31, 39, 46, 2, 45, 20, 47, 42, 38, 50, 58, 33, 43, 44, 59, 49, 40, 48, 52, 56, 53, 55, 54, 61, 64, 62, 63, 57, 51, 69, 68, 72, 82, 60, 65, 66, 75, 79, 67, 70, 71, 76, 78, 73, 81, 74, 77, 83, 84, 88, 90, 91, 87, 80, 99 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
A permutation of the positive integers.
LINKS
EXAMPLE
a(1) = 1 is the least positive integer, and a(1) has no other constraint to satisfy.
a(2) = 9 is the least positive integer > a(1) = 1 such that a(2)*a(1) = 9 has a digit 9.
a(3) = 10 is the least positive integer not in {1, 9} such that a(3)*a(2) (= 90) has a digit 9: The smaller choices 2, ..., 8 does not satisfy this.
a(4) = 19 is the least positive integer not in {1, 9, 10} such that a(4)*a(3) (= 190) has a digit 5: All available smaller choices do not satisfy this.
PROG
(PARI) A298979(n, f=1, d=9, 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}
CROSSREFS
Cf. A299980, A299981, A299402, A299403, A298974, A298975, A299996, A299997, A298978 : analog with digit 0, 1,..., 8.
Cf. A299957, A299969, ..., A299988 (analog with addition instead of multiplication, and different digits).
Sequence in context: A260042 A141640 A231504 * A299969 A050551 A022099
KEYWORD
nonn,base
AUTHOR
M. F. Hasler, Feb 22 2018
STATUS
approved

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Last modified March 29 08:01 EDT 2024. Contains 371265 sequences. (Running on oeis4.)