OFFSET
1,1
COMMENTS
Even terms in A060462.
And A071904 are the successors of a(n).
Even numbers that are not a prime - 1. That is, even numbers not in A006093. - Terry D. Grant, Oct 31 2020
REFERENCES
J. D. E. Konhauser et al., Which Way Did The Bicycle Go?, Problem 98, pp. 29; 145-146, MAA Washington DC, 1996.
Die WURZEL - Zeitschrift für Mathematik, 53. Jahrgang, Juli 2019, S. 171, WURZEL-Aufgabe 2019-36 von Gerhard Dietel, Regensburg.
FORMULA
a(n) = A071904(n) - 1.
EXAMPLE
8! = 40320 is divisible by 8*9/2 = 36.
14! is divisible by 14*15/2.
MATHEMATICA
Complement[Table[2 n, {n, 1, 103}], Table[EulerPhi[Prime[n]], {n, 1, 103}]] (* Terry D. Grant, Oct 31 2020 *)
PROG
(PARI) forcomposite(c=4, 10^3, if(c%2==1, print1(c-1, ", "))); \\ Joerg Arndt, Jul 25 2019
(Magma) [k: k in [2..250]|IsEven(k) and Factorial(k) mod Binomial(k+1, 2) eq 0]; // Marius A. Burtea, Jul 28 2019
(Python)
from sympy import primepi
def A309355(n):
if n == 1: return 8
m, k = n, primepi(n) + n + (n>>1)
while m != k:
m, k = k, primepi(k) + n + (k>>1)
return m-1 # Chai Wah Wu, Aug 02 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
Gerhard Palme, Jul 25 2019
STATUS
approved