# How to use a pure function to write following sum?

How to use a pure function to write following sum? I have tried to use FoldList, but it is still not working. • p = Total[(Rest@FoldList[Plus, 0, Range[#]])^-1] & – cvgmt May 4 at 15:12
• it is not correct when i use n=10 – Udf Hx May 4 at 15:23
• If you clear any old definitions (Clear["Global*"]), solution provided by @cvgmt works as expected. – Bob Hanlon May 4 at 17:14

Try RSolve

\[Rho] = RSolveValue[{rho[n] - rho[n - 1] == 1/Sum[i, {i, 1, n}], rho == 1}, rho, n]
(*Function[{n}, (2 n)/(1 + n)]*)

\[Rho]
(*20/11*)


One does not need to invent something complicated, just write what you see

f[n_] := Sum[1/Sum[k, {k, m}], {m, n}]
f
(*20/11*)


and in pure form

g = Sum[1/Sum[k, {k, m}], {m, #}] &
g@10
g@t
(*20/11*)
(*2 - 2/(1 + t)*)

p = Function[n, Sum[2/k/(k + 1), {k, 1, n}]];
p (* 20/11 *)


If you are really committed to using Fold, you could try:

p = Function[n, Last@Fold[
With[{dn = #1[] + #2}, {dn, #1[] + 1/dn}] &,
{0, 0}, GeneralUtilities$$$$RangeIterator[n]]]