login
This site is supported by donations to The OEIS Foundation.

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A298974 Lexicographic first sequence of positive integers such that a(n)*a(n+1) has a digit 4, and no term occurs twice. 15
1, 4, 6, 7, 2, 12, 17, 20, 21, 14, 3, 8, 5, 9, 16, 15, 23, 18, 13, 11, 22, 19, 24, 10, 34, 26, 29, 36, 39, 32, 27, 35, 40, 31, 37, 38, 28, 30, 47, 42, 44, 33, 43, 48, 50, 49, 46, 51, 54, 41, 45, 52, 57, 25, 56, 58, 53, 65, 62, 55, 59, 60, 64, 61, 63, 66, 67, 68, 69, 70, 71, 74, 73, 75, 72, 76, 79, 82, 77, 84, 81, 80, 78, 83, 88, 85, 97, 86, 87 (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) = 4 is the least positive integer > a(1) = 1 such that a(2)*a(1) = 4 has a digit 4.

a(3) = 6 is the least positive integer not in {1, 4} such that a(3)*a(2) (= 24) has a digit 4: The smaller choices 2, 3 and 5 do not satisfy this.

a(4) = 7 is the least positive integer not in {1, 4, 6} such that a(4)*a(3) (= 42) has a digit 4: All available smaller choices do not satisfy this.

PROG

(PARI) A298974(n, f=1, d=4, 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. A299402, A299403, A298975, ..., A298979: analog with digit 2, 3; ..., 9.

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

Sequence in context: A135798 A197010 A021218 * A237515 A200939 A244816

Adjacent sequences:  A298971 A298972 A298973 * A298975 A298976 A298977

KEYWORD

nonn,base

AUTHOR

M. F. Hasler, Feb 22 2018

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified December 15 14:37 EST 2019. Contains 329999 sequences. (Running on oeis4.)