OFFSET
1,3
COMMENTS
For each of the terms, the number of ternary (= base 3) digits is a triangular number A000217.
The base 2 analog would have only the 5 terms 0,1,4,5,6. See A218556 for the base 10 analog.
The sequence A167819 is a subsequence containing exactly all terms >= 9.
The sequence is finite, with 255=3+12+240 (= 1 + sum of the 3rd row of A218566) terms.
LINKS
M. F. Hasler, Table of n, a(n) for n = 1..255 (full sequence).
EXAMPLE
The terms a(1)=0 through a(3)=2 have exactly 1 digit occurring exactly once.
The terms a(4)=9=100[3] through a(15)=25=221[3], have one ternary digit occurring once and a second, different digit occurring exactly twice.
The terms a(16)=248=100012[3] through a(255)=714=222110[3] contain each ternary digit at least once. There are no other terms in this sequence.
PROG
(PARI) {my(T(n)=n*(n+1)\2); print1(0); for(i=1, 3, s=vector(i+1, j, j-1); for(n=3^(T(i)-1), 3^T(i)-1, i !=#Set(digits(n, 3)) & next; c=vector(4); for(j=1, #d=digits(n, 3), c[d[j]+1]++); vecsort(c, , 8)==s & print1(", "n)))}
(PARI) is_A218560(n, b=3)={ my(c=vector(b+1)); for(i=1, #n=digits(n, b), c[n[i]+1]++); #(c=vecsort(c, , 8))==1+c[#c] && 2*#n==c[#c]*#c }
CROSSREFS
KEYWORD
nonn,easy,base,fini,full
AUTHOR
M. F. Hasler, Nov 02 2012
STATUS
approved