

A134661


Number of odd coefficients in (1 + x + x^3)^n.


5



1, 3, 3, 7, 3, 9, 7, 13, 3, 9, 9, 19, 7, 21, 13, 27, 3, 9, 9, 21, 9, 27, 19, 35, 7, 21, 21, 41, 13, 39, 27, 55, 3, 9, 9, 21, 9, 27, 21, 39, 9, 27, 27, 55, 19, 57, 35, 73, 7, 21, 21, 49, 21, 63, 41, 75, 13, 39, 39, 79, 27, 81, 55, 109, 3, 9, 9, 21, 9, 27, 21, 39, 9, 27, 27, 57, 21, 63, 39
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OFFSET

0,2


LINKS



EXAMPLE

Written as an irregular triangle in which the row lengths are the terms of A011782, the sequence begins:
1;
3;
3,7;
3,9,7,13;
3,9,9,19,7,21,13,27;
3,9,9,21,9,27,19,35,7,21,21,41,13,39,27,55;
3,9,9,21,9,27,21,39,9,27,27,55,19,57,35,73,7,21,21,49,21,63,41,75,13,39,39,79,27,81,55,109;
3,9,9,21,9,27,21,39,9,27,27,57,21,63,39...
...
Note that in each row a fraction of the first terms are equal to 3 times the beginning of the sequence itself. For rows 06 the fractions are: 0, 1, 1/2, 1/2, 3/8, 3/8, 11/32. Apparently the fractions converge to a constant.
(End)


MATHEMATICA

PolynomialMod[(1+x+x^3)^n, 2] /. x>1


PROG

(PARI) a(n) = {my(pol= Pol([1, 0, 1, 1], xx)*Mod(1, 2)); subst(lift(pol^n), xx, 1); } \\ Michel Marcus, Mar 01 2015


CROSSREFS



KEYWORD

nonn


AUTHOR



STATUS

approved



