Find the number of all subsets of $\{1, 2, \ldots,2015\}$ with $n$ elements such that the sum of the elements in the subset is divisible by 5

A generating function solution.

For every $S\subset\{1,2,\ldots,2015\}$ we will write $\Sigma S=\sum_{k\in S}k$.

Let $$ f(a,x) = \prod_{k=1}^{2015} (1+a^kx) = \sum_{S\subset\{1,\ldots,2015\}} a^{\Sigma S} x^{|S|}. $$ Take the average this function over putting $5$th complex roots of unity for $a$. Let $\omega=e^{2\pi i/5}$; then $$ \frac15\sum_{j=0}^4 f(\omega^j,x) = \sum_{S\subset\{1,\ldots,2015\}} \left(\frac15\sum_{j=0}^4 \big(\omega^{\Sigma S}\big)^j\right) x^{|S|} = \sum_{\substack{S\subset\{1,\ldots,2015\}\\\Sigma S\equiv0\pmod5}} x^{|S|}. \tag{$*$} $$ On the RHS of $(*)$, the coefficient of $x^n$ is the number of $n$-element sets $S\subset\{1,\ldots,2015\}$ with $\Sigma S\equiv0\pmod5$.

On the other hand, $$ f(\omega^j,x)= \begin{cases} (1+x)^{2015} & \text{if } j=0 \\ (1+x^5)^{403} & \text{if } j=1,2,3,4 \end{cases} $$ so on the LHS of $(*)$ the coefficient of $x^n$ is: $\frac15\binom{2015}n$ if $n$ is co-prime with $5$, and $\frac15\binom{2015}n+\frac45\binom{403}{n/5}$ if $5|n$.

This is a straightforward application of the Polya Enumeration Theorem. We treat the problem of subsets with $n$ elements of the set $\{1,2,\ldots, q\}$ whose sum is divisible by $k.$ Suppose $Z(P_n)$ is the cycle index of the set operator $\mathfrak{P}_{=n}$ given by the recurrence by Lovasz which is

$$Z(P_n) = \frac{1}{n} \sum_{l=1}^n (-1)^{l+1} a_l Z(P_{n-l}) \quad\text{where}\quad Z(P_0) = 1.$$

We obtain by PET the following formula for the OGF of ordinary sets

$$Z(P_n)\left(w+w^2+\cdots+w^q\right) = Z(P_n)\left(\sum_{m=1}^q w^m\right).$$

With $\rho$ a root of unity namely $$\rho = \exp(2\pi i/k)$$

we get for the desired count the value $$\frac{1}{k} \left.\sum_{p=0}^{k-1} Z(P_n)\left(\sum_{m=1}^q w^m\right)\right|_{w=\rho^p}.$$

We will compute the value for $p=0$ separately and to do this recall the OGF of the set operator $\mathfrak{P}_{=n}$ which is

$$Z(P_n) = [z^n] \exp\left(a_1 z - a_2 \frac{z^2}{2} + a_3 \frac{z^3}{3} - a_4 \frac{z^4}{4} +\cdots \right).$$

or

$$Z(P_n) = [z^n] \exp\left(\sum_{r\ge 1} (-1)^{r+1} a_r \frac{z^r}{r}\right).$$

On substituting this into our formula we get

$$\frac{1}{k} \sum_{p=0}^{k-1} \left. [z^n] \exp\left(\sum_{r\ge 1} (-1)^{r+1} \frac{z^r}{r} \sum_{m=1}^q w^{rm} \right) \right|_{w=\rho^p}.$$

When $p=0$ we obtain $$\frac{1}{k} [z^n] \exp\left(q\sum_{r\ge 1} (-1)^{r+1} \frac{z^r}{r}\right) = \frac{1}{k} [z^n] \exp\left(q \log (1+z)\right) \\ = \frac{1}{k} [z^n] (1+z)^q = \frac{1}{k} {q\choose n}.$$

We switch to algorithmics for the remainder of this discussion.

In treating the case $p\ge 1$ we make the following observation. When substituting into the terms of the cycle index those $a_r$ from the product where $pr$ is a multiple of $k$ produce the value $q$ while the remaining $a_r$ create a sequence of period $k$ that depends only on the remainder $b$ when $q$ is divided by $k$ where we take $1\le b\le k.$

This yields an algorithm where we iterate over the cycle index, extract eventual powers of $q$ from the terms and interpolate the rest in terms of $b.$ The algorithm can be used to compute formulae for fixed combinations of $n$ and $k$ like the ones at this MSE link automatically.

We obtain for $(n,k) = (3,3)$ $$1/18\,{q}^{3}+1/3\,{b}^{2}-1/6\,{q}^{2}-{\frac {11\,b}{9}}+q/3+2/3$$ and sure enough comparing it to the link these are the right values.

Supposing now that we are interested in divisibility by five of three-element subsets i.e. the pair $(n,k) = (3,5)$ we find

$$1/12\,{b}^{4}-{\frac {13\,{b}^{3}}{15}}+1/30\,{q}^{3}+{\frac { 181\,{b}^{2}}{60}}-1/10\,{q}^{2}-{\frac {127\,b}{30}}+q/15+2$$

which gives the sequence (starting at $q=3$)

$$0, 0, 2, 4, 7, 11, 16, 24, 33, 44, 57, 72, 91, \ldots$$

For the pair $(11,5)$ we obtain

$${\frac {{q}^{11}}{199584000}}-{\frac {{q}^{10}}{3628800}}+{ \frac {{q}^{9}}{151200}}-{\frac {11\,{q}^{8}}{120960}}+{\frac { 683\,{q}^{7}}{864000}}+{\frac {{b}^{4}{q}^{2}}{1200}}\\-{\frac { 781\,{q}^{6}}{172800}}-{\frac {{b}^{4}q}{80}}-{\frac {{b}^{3}{q }^{2}}{120}}+{\frac {31063\,{q}^{5}}{1814400}}+1/24\,{b}^{4}+1/ 8\,{b}^{3}q+{\frac {7\,{b}^{2}{q}^{2}}{240}}\\-{\frac {1529\,{q}^ {4}}{36288}}-{\frac {631\,{b}^{3}}{1500}}-{\frac {859\,{b}^{2}q }{2000}}-{\frac {137\,b{q}^{2}}{3000}}+{\frac {16103\,{q}^{3}}{ 252000}}+{\frac {863\,{b}^{2}}{600}}\\+{\frac {129\,bq}{200}}-{ \frac {419\,{q}^{2}}{12600}}-{\frac {25\,b}{12}}-{\frac {31\,q }{110}}+1$$

which gives the sequence (starting at $q=11$)

$$0, 2, 15, 72, 273, 873, 2474, 6363, 15114, 33592, \ldots.$$

Another interesting pair is $(3,6)$ which gives

$$-{\frac {{b}^{5}q}{90}}-{\frac {{b}^{5}}{90}}+{\frac {7\,{b}^{4 }q}{36}}+1/4\,{b}^{4}-{\frac {23\,{b}^{3}q}{18}}-2\,{b}^{3}+{ \frac {35\,{b}^{2}q}{9}}\\+1/36\,{q}^{3}+7\,{b}^{2}-{\frac {242\, bq}{45}}-1/12\,{q}^{2}-{\frac {313\,b}{30}}+{\frac {17\,q}{6}}+5$$

and $(4,7)$ which produces

$${\frac {{b}^{6}}{360}}-{\frac {3\,{b}^{5}}{40}}+{\frac {389\,{b }^{4}}{504}}+{\frac {{q}^{4}}{168}}-{\frac {215\,{b}^{3}}{56}}- 1/28\,{q}^{3}\\+{\frac {3041\,{b}^{2}}{315}}+{\frac {11\,{q}^{2} }{168}}-{\frac {403\,b}{35}}-q/28+5.$$

The Maple code for this including a total enumeration routine for verification and some code to prettify the formulae for $k$ small was as follows.

with(combinat);

pet_cycleind_set :=
proc(n)
option remember;

    if n=0 then return 1; fi;

    expand(1/n*add((-1)^(l+1)*a[l]*pet_cycleind_set(n-l), l=1..n));
end;

pet_flatten_term :=
proc(varp)
local terml, d, cf, v;

    terml := [];

    cf := varp;
    for v in indets(varp) do
        d := degree(varp, v);
        terml := [op(terml), seq(v, k=1..d)];
        cf := cf/v^d;
    od;

    [cf, terml];
end;

V :=
proc(q, n, k)
    local comb, res;
    option remember;

    res := 0;
    comb := firstcomb(q, n);

    while type(comb, set) do
        if add(p, p in comb) mod k = 0 then
            res := res + 1;
        fi;

        comb := nextcomb(comb, q);
    od;

    res;
end;

CF :=
proc(n, k)
    local term, rho, res, p, flat, ixvar, rmd, m,
    rep, qpow, w, ex, vals, val;
    option remember;

    res := 0; rho := exp(2*Pi*I/k);

    for term in pet_cycleind_set(n) do
        flat := pet_flatten_term(term);

        for p to k-1 do
            vals := []; rep := []; qpow := 0;

            for ixvar in flat[2] do
                ex := p*op(1, ixvar);

                if ex mod k = 0 then
                    qpow := qpow + 1;
                else
                    rep := [op(rep), ixvar];
                fi;
            od;


            for rmd to k do
                val := 1;

                for ixvar in rep do
                    ex := p*op(1, ixvar);
                    val := val * add(w^(ex*m), m=1..rmd);
                od;

                vals :=
                [op(vals), subs(w=rho, expand(val))];
            od;

            res := res + flat[1]*q^qpow *
            interp([seq(b, b=1..k)], vals, b);
        od;
    od;

    binomial(q,n)/k + res/k;
end;

CFsimp :=
proc(n, k)
    local form, res, term, lcf, cnst;
    option remember;

    form := collect(expand(CF(n,k)), {b,q}, `distributed`);

    cnst := coeff(coeff(form, b, 0), q, 0);

    res := 0;

    for term in form-cnst do
        lcf := lcoeff(term);

        res := res +
        simplify(Re(lcf))*term/lcf;
    od;

    res + simplify(cnst);
end;

F :=
proc(qv, n, k)
local bv;

    if qv mod k = 0 then
        bv := k;
    else
        bv := qv mod k;
    fi;

    subs({q=qv, b=bv}, CFsimp(n,k));
end;

The reader is invited to contribute a better simplification routine making more effective use of the mathematical givens. The Maple code should be considered betaware.

Remark Sat Jan 23 2016. I present one of the special cases where radical simplification is possible. Start from the formula

$$\frac{1}{k} {q\choose n} + \frac{1}{k} \sum_{p=1}^{k-1} \left. [z^n] \exp\left(\sum_{r\ge 1} (-1)^{r+1} \frac{z^r}{r} \sum_{m=1}^q w^{rm} \right) \right|_{w=\rho^p}.$$

Now suppose that $q$ is a multiple of $k$ and $k$ is an odd prime. Observe that the sum $$\sum_{m=1}^q w^{rm}$$ is equal to $q/k\times k = q$ if $pr$ is a multiple of $k$ and zero otherwise. But $pr$ can only be a multiple of $k$ if $r$ is a multiple of $k.$ This yields

$$\frac{1}{k} {q\choose n} + \frac{1}{k} \sum_{p=1}^{k-1} \left. [z^n] \exp\left(\sum_{r\ge 1} (-1)^{kr+1} \frac{z^{kr}}{kr} \sum_{m=1}^q w^{krm} \right) \right|_{w=\rho^p} \\ = \frac{1}{k} {q\choose n} + \frac{1}{k} \sum_{p=1}^{k-1} \left. [z^n] \exp\left(\sum_{r\ge 1} (-1)^{kr+1} \frac{z^{kr}}{kr} \frac{q}{k}\times k \right) \right|_{w=\rho^p} \\ = \frac{1}{k} {q\choose n} + \frac{1}{k} \sum_{p=1}^{k-1} [z^n] \exp\left(\frac{q}{k} \sum_{r\ge 1} (-1)^{kr+1} \frac{z^{kr}}{r}\right) \\ = \frac{1}{k} {q\choose n} + \frac{1}{k} \sum_{p=1}^{k-1} [z^n] \exp\left(-\frac{q}{k} \sum_{r\ge 1} \frac{(-z)^{kr}}{r}\right) \\ = \frac{1}{k} {q\choose n} + \frac{1}{k} \sum_{p=1}^{k-1} [z^n] \exp\left(-\frac{q}{k} \log\frac{1}{1-(-z)^k}\right) \\ = \frac{1}{k} {q\choose n} + \frac{1}{k} \sum_{p=1}^{k-1} [z^n] (1+z^k)^{q/k}.$$

Therefore if $n$ is coprime to $k$ we obtain $$\frac{1}{k} {q\choose n}$$

and if it is a multiple of $k$

$$\frac{1}{k} {q\choose n} + \frac{k-1}{k} {q/k\choose n/k}.$$

The answer is $\frac{1}{5}\binom{2015}{n}$.

For each subset of $n$ elements (not necessarily with sum multiple of $5$) consider the set of the $2015$ translations of the $n$-set. In other words, if we have $\{a_1,a_2\dots a_n\}$ Consider the family of sets of the form $\{r(a_1+k),r(a_2+k),\dots r(a_3+k)\}$ with $k\in\{0,1,\dots 2015\}$, and where $r(m)$ is simply the smallest positive integer congruent to $m\bmod 2015$.

It should be clear this splits the $n-sets$ into $\binom{2015}{n}/2015$ families, each containing $2015$ sets.

Moreover, the sum of the sets in the familiy of $\{a_1,a_2\dots a_{2015}\}$ are congruent to:

$S,S+n,S+2n,\dots ,S+2015n$, where $S=a_1+a_2+\dots + a_n$

Because $n$ is coprime with $5$ we deduce $\{S,S+n,S+2n\dots S+4n\}$ is a complete reside system $\bmod 5$, as well as $\{S+5n,S+6n,S+7n,S+8n, S+9n\}$ etcetera. Because of this, exactly $2015/5$ of the sets of each family have sums which are multiples of $5$. The result follows.