OFFSET
0,2
REFERENCES
Steven R. Finch, Mathematical Constants II, Encyclopedia of Mathematics and Its Applications, Cambridge University Press, Cambridge, 2018, p. 166.
LINKS
Amiram Eldar, Table of n, a(n) for n = 0..10000
FORMULA
a(n) ~ (H/4) * n^3, where H = Product_{p prime, p == 1 (mod 4)} (1 - 2/p^2) = 0.8948412245... (A335963).
EXAMPLE
a(0) = phi(0^2 + 1) = phi(1) = 1.
a(1) = phi(0^2 + 1) + phi(1^2 + 1) = phi(1) + phi(2) = 1 + 1 = 2.
MATHEMATICA
Accumulate @ Table[EulerPhi[k^2 + 1], {k, 0, 100}]
PROG
(PARI) a(n) = sum(k=0, n, eulerphi(k^2+1)); \\ Michel Marcus, Mar 10 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Amiram Eldar, Mar 09 2020
STATUS
approved