Btukfyl

I am trying to draw a tree using tikzpicture like this:

begin{tikzpicture}

    tikzstyle{node}=[circle, fill=blue!25, minimum size=0.1 cm];

    foreach ilayer in {0,...,3} {

        tikzmath {nnodes = 3 ^ ilayer; }

        tikzmath {leftnum = 1 - floor(nnodes / 2) - 1; }

        tikzmath {rightnum = nnodes - floor(nnodes / 2) - 1; }

        foreach isibling in {leftnum,...,rightnum} {                

            tikzmath {d = 3 ^ (- ilayer) * 15; }

            tikzmath {x = isibling * d; }

            tikzmath {y = - ilayer * 2; }

            node[node] (node_ilayer_isibling) at (x cm, y cm) {isibling};

        }

    }

end{tikzpicture}

I get the tree like the following picture. The texts are nodes' isibling within each layer. Most nodes are integers, but all leftmost nodes are not.

You could just tell TikZ explicitly that you want an integer.

documentclass[tikz,border=3.14mm]{standalone}

usetikzlibrary{math}

begin{document}

begin{tikzpicture}

    tikzset{mynode/.style={circle, fill=blue!25, minimum size=0.1 cm}}

    foreach ilayer in {0,...,3} {

        tikzmath {nnodes = 3 ^ ilayer; }

        tikzmath {leftnum = int(1 - floor(nnodes / 2) - 1); }

        tikzmath {rightnum = nnodes - floor(nnodes / 2) - 1; }

        foreach isibling in {leftnum,...,rightnum} {                

            tikzmath {d = 3 ^ (- ilayer) * 15; }

            tikzmath {x = isibling * d; }

            tikzmath {y = - ilayer * 2; }

            node[mynode] (node_ilayer_isibling) at (x cm, y cm) {isibling};

        }

    }

end{tikzpicture}

end{document}

Or

documentclass[tikz,border=3.14mm]{standalone}

usetikzlibrary{math}

begin{document}

begin{tikzpicture}

    tikzset{mynode/.style={circle, fill=blue!25, minimum size=0.1 cm}}

    foreach ilayer in {0,...,3} {

        tikzmath {int nnodes,leftnum,rightnum;

            nnodes = 3 ^ ilayer;

            leftnum = 1 - floor(nnodes / 2) - 1; 

        rightnum = nnodes - floor(nnodes / 2) - 1; }

        foreach isibling in {leftnum,...,rightnum} {                

            tikzmath {d = 3 ^ (- ilayer) * 15; 

            x = isibling * d; 

            y = - ilayer * 2; }

            node[mynode] (node_ilayer_isibling) at (x cm, y cm) {isibling};

        }

    }

end{tikzpicture}

end{document}

In principle you do not need the math library here.

documentclass[tikz,border=3.14mm]{standalone}

begin{document}

begin{tikzpicture}

    tikzset{mynode/.style={circle, fill=blue!25, minimum size=0.1 cm}}

    foreach ilayer [evaluate=ilayer as nnodes using {int(3 ^ ilayer)},

    evaluate=nnodes as leftnum using {int(1 - floor(nnodes / 2) - 1)},

    evaluate=nnodes as rightnum using {int(nnodes - floor(nnodes / 2) - 1)}]

    in {0,...,3} {

        foreach isibling 

        [evaluate=ilayer as d using {3 ^ (- ilayer) * 15},

        evaluate=isibling as x using {isibling * d},

        evaluate=ilayer as y using {- ilayer * 2}]

        in {leftnum,...,rightnum} {                

            node[mynode] (node_ilayer_isibling) at (x cm, y cm) 

            {isibling};

        }

    }

end{tikzpicture}

end{document}