OFFSET
1,1
COMMENTS
a(n) > n.
Numbers n > 1 such that a(n-1) = n are listed in A015950.
a(87) > 10^14.
a(11) <= 2791496231, a(17) <= 140631956671, a(53) <= 52134328061 from Joe K. Crump (joecr(AT)carolina.rr.com), Feb 10 2007
LINKS
Ryan Propper, Table of n, a(n) for n = 1..82
FORMULA
a(5^k-1) = 5^k.
MATHEMATICA
Do[k = 1; While[PowerMod[4, k, k] != n, k++ ]; Print[k], {n, 30}]
t = Table[0, {10000} ]; k = 1; While[ k < 5000000000, a = PowerMod[4, k, k]; If[a < 10001 && t[[a]] == 0, t[[a]] = k; Print[{a, k}]]; k++ ]; t (* search limits expanded by Robert G. Wilson v, Jul 14 2009 *)
PROG
(Python)
def a(n):
k = 1
while 4**k % k != n: k += 1
return k
print([a(n) for n in range(1, 11)]) # Michael S. Branicky, Mar 14 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Ryan Propper, Jun 12 2006
EXTENSIONS
a(11) = 2791496231 from Robert G. Wilson v, Feb 11 2007; confirmed by Ryan Propper, Feb 15 2007
Link corrected by R. J. Mathar, Jul 24 2009
a(83) = 3085807457009 = 113 * 331 * 82501603 from Hagen von Eitzen, Jul 27 2009
STATUS
approved