OFFSET
1,1
COMMENTS
Guy, in his Example 8, citing Leo Moser as his source, noted that the first 9 values of A005728(n) = 1 + Sum_{i=1..n} phi(i) = 1 + Sum_{i=1..n} A000010(i) are all primes, but that the pattern breaks down at A005728(10) = 33 = 3*11. As Guy warns, in several paraphrases of the same law, "Capricious coincidences cause careless conjectures." That is, for 1 <= n <= 9 we have A005728(n) = A078334(n), but for n > 9 we sometimes (n = {11, 12, 13, 15, 17, 18, 22, ...}) have A005728(n) prime, but other times (n = {10, 14, 16, 19, 20, 21, ...}) have A005728(n) composite. [Jonathan Vos Post, Sep 06 2010]
REFERENCES
H. Rademacher, Lectures on Elementary Number Theory, 1964. pp. 5-11.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
Richard K. Guy, The Strong Law of Small Numbers, Amer. Math. Monthly 95 (1988), no. 8, 697-712.
Eric Weisstein's World of Mathematics, Farey Sequence.
EXAMPLE
The Farey sequence of order 6 is {0, 1/6, 1/5, 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 1}, which has 13 terms, so 13 is in the sequence.
MATHEMATICA
fc[n_] := 1+Sum[EulerPhi[k], {k, 1, n}]; Select[fc/@Range[200], PrimeQ]
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Cino Hilliard, Nov 21 2002
EXTENSIONS
Offset corrected by Amiram Eldar, Mar 01 2020
STATUS
approved