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A211186 Smallest strictly increasing sequence such that no term is the sum of any two digits of the sequence. 1
1, 2, 4, 7, 44, 47, 48, 49, 74, 77, 78, 79, 84, 87, 88, 89, 94, 97, 98, 99, 444, 447, 448, 449, 474, 477, 478, 479, 484, 487, 488, 489, 494, 497, 498, 499, 744, 747, 748, 749, 774, 777, 778, 779, 784, 787, 788, 789, 794, 797, 798, 799, 844, 847, 848, 849, 874, 877 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

"Smallest" means lexicographically (1 < 2 < 10 < ...) first.

From a(2)=2 on, there may not occur any other term with the digit 1. From a(3)=4 on, the digits 2 and 3 are excluded. From a(4)=7 on, the digits 5 and 6 are excluded. From a(5)=44 on, the digit 0 is also excluded, and subsequent terms are all larger numbers made from digits 4,7,8 or 9; one can check that then no further contradictions can appear.

Thus there are 4^d terms with d digits, for d=1,2,3,... This leads to an explicit formula for the n-th term.

LINKS

Table of n, a(n) for n=1..58.

E. Angelini, Post to the SeqFan mailing list, Feb 02 2013

PROG

(PARI) A211186(n)={n>4 & for(d=1, n--, n < 4^d & return(sum(k=1, d, [4, 7, 8, 9][n%4+1]*10^(k-1)+0*n\=4)); n -= 4^d); [1, 2, 4, 7][n]}

CROSSREFS

Sequence in context: A082537 A004577 A285940 * A302938 A076719 A103009

Adjacent sequences:  A211183 A211184 A211185 * A211187 A211188 A211189

KEYWORD

nonn,base

AUTHOR

Eric Angelini and M. F. Hasler, Feb 02 2013

STATUS

approved

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Last modified January 16 23:44 EST 2019. Contains 319206 sequences. (Running on oeis4.)