# 4 Positive Integers in Arithmetic Progression which have Same Euler Phi Value

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## Theorem

The following sets of $4$ positive integers which form an arithmetic progression are the smallest which all have the same Euler $\phi$ value:

- $72, 78, 84, 90$
- $216, 222, 228, 234$
- $76 \, 236, 76 \, 240, 76 \, 246, 76 \, 252$

## Proof

\(\displaystyle 78 = 72\) | \(=\) | \(\displaystyle 6\) | |||||||||||

\(\displaystyle 84 - 78\) | \(=\) | \(\displaystyle 6\) | |||||||||||

\(\displaystyle 90 - 84\) | \(=\) | \(\displaystyle 6\) |

demonstrating that this is indeed an arithmetic progression, with a common difference of $6$.

Now we show:

\(\displaystyle \map \phi {72}\) | \(=\) | \(\displaystyle 24\) | Euler Phi Function of 72 | ||||||||||

\(\displaystyle \map \phi {78}\) | \(=\) | \(\displaystyle 24\) | Euler Phi Function of 78 | ||||||||||

\(\displaystyle \map \phi {84}\) | \(=\) | \(\displaystyle 24\) | Euler Phi Function of 84 | ||||||||||

\(\displaystyle \map \phi {90}\) | \(=\) | \(\displaystyle 24\) | Euler Phi Function of 90 |

## Sources

- Apr. 1972: M. Lal and P. Gillard:
*On the Equation $\map \phi n = \map \phi {n + k}$*(*Math. Comp.***Vol. 26**,*no. 118*: 579 – 583) www.jstor.org/stable/2005186

- 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $72$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $72$