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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

Table of n, a(n) for n=1..89.

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

Adjacent sequences:  A298976 A298977 A298978 * A298980 A298981 A298982

KEYWORD

nonn,base

AUTHOR

M. F. Hasler, Feb 22 2018

STATUS

approved

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Last modified November 17 11:02 EST 2019. Contains 329226 sequences. (Running on oeis4.)