# Harmonic Numbers/Examples/H10000

## Example of Harmonic Number

To $15$ decimal places:

$H_{10000} \approx 9 \cdotp 78760 \, 60360 \, 44382 \, \ldots$

where $H_{10000}$ denotes the $10 \, 000$th harmonic number.

## Proof

 $\ds H_{10000}$ $\approx$ $\ds \ln 10 \, 000 + \gamma + \dfrac 1 {2 \times 10000} - \dfrac 1 {12 \times \left({10000}\right)^2} + \dfrac 1 {12 \times \left({10000}\right)^4} + \epsilon$ Approximate Size of Sum of Harmonic Series $\ds$ $=$ $\ds 4 \ln 10 + \gamma + \dfrac 1 {2 \times 10000} - \dfrac 1 {12 \times \left({10000}\right)^2} + \dfrac 1 {12 \times \left({10000}\right)^4} + \epsilon$ Logarithm of Power

$\blacksquare$

We have:

 $\ds \ln 10$ $\approx$ $\ds 2 \cdotp 30258 \, 50929 \, 94045 \, 68401 \, 7991 \ldots$ Natural Logarithm of 10 $\ds \gamma$ $\approx$ $\ds 0 \cdotp 57721 \, 56649 \, 01532 \, 86060 \, 65120 \ldots$ Definition of Euler-Mascheroni Constant $\ds \dfrac 1 {2 \times 10000}$ $=$ $\ds 0 \cdotp 00005$ Definition of Euler-Mascheroni Constant $\ds \dfrac 1 {12 \times 10000^2}$ $\approx$ $\ds 0 \cdotp 00000 \, 00008 \, 33333 \, 33333$ $\ds \dfrac 1 {120 \times 10000^4}$ $<$ $\ds 10^{-18}$

Thus for an accuracy of $15$ decimal places it is unnecessary to consider $\dfrac 1 {120 \times 10000^4}$ and smaller terms.

Then:

  2.30258 50929 94045 68401
x                         4
-------------------------
9.21034 03719 76182 73604
+ 0.57721 56649 01532 86060
+ 0.00005
-------------------------
9.78760 50368 77715 59664
- 0.00000 00008 33333 33333
-------------------------
9.78760 50360 44382 26331


Hence the result.

$\blacksquare$