

A064686


a(n) = number of ndigit base3 biquams.


5



0, 2, 7, 23, 73, 227, 697, 2123, 6433, 19427, 58537, 176123, 529393, 1590227, 4774777, 14332523, 43013953, 129074627, 387289417, 1161999323, 3486260113, 10459304627, 31378962457, 94138984523, 282421147873, 847271832227
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OFFSET

1,2


COMMENTS

A biquam or biquanimous number (A064544) is a number whose digits can be split into two groups with equal sum.
This is the same as A083313 (apart from the initial term). Proof: Let sum(w) denote the sum of the digits of w. There are 2*3^(n1) ndigit base3 numbers: w = (w_1,w_2,...,w_n) with w_i in {0,1,2} for all i and w_1 != 0. Partition them into 4 classes: (i) sum(w) is odd, (ii) sum(w) is even, w contains no 1's and has an odd number of 2s, (iii) sum(w) is even, w contains no 1's and has an even number of 2s and (iv) sum(w) is even and w contains some 1's. Clearly, no biquams occur in cases (i) and (ii), case (iii) consists entirely of biquams and, we claim, so does case (iv). For case (iv) forces an even number, say 2k, of 1's. An even number of 2s clearly gives a biquam and an odd number 2m+1 of 2s does too because {m 2s, (k+1) 1's} and {(m+1) 2s, (k1) 1's} is a biquam split. There are 3^(n1) w's in case (i) and 2^(n2) w's in case (ii) and hence 2*3^(n1)  (3^(n1) + 2^(n2)) = 3^(n1)  2^(n2) (A083313) biquams among ndigit base3 numbers.  David Callan, Sep 15 2004
a(n) % 100 = 23 for n = 4*k1, k>=1; a(n) % 100 = 27 for n = 4*k+1, k>=1.  Alex Ratushnyak, Jul 03 2012


LINKS

Table of n, a(n) for n=1..26.
Index entries for linear recurrences with constant coefficients, signature (5,6).


FORMULA

a(1) = 0, a(n) = 3^(n1)2^(n2) for n>=2.  Alex Ratushnyak, Jul 02 2012
a(n) = 5*a(n1)6*a(n2) for n>3. G.f.: x^2*(3*x2) / ((2*x1)*(3*x1)).  Colin Barker, May 27 2013


PROG

(Python)
print([0]+[3**n  2**(n1) for n in range(1, 29)])
# Alex Ratushnyak, Jul 02 2012


CROSSREFS

Essentially the same as A083313.
Cf. A053152 (partial sums).
Sequence in context: A295136 A192906 A217664 * A083313 A077832 A030282
Adjacent sequences: A064683 A064684 A064685 * A064687 A064688 A064689


KEYWORD

base,easy,nonn


AUTHOR

David W. Wilson, Oct 10 2001


STATUS

approved



