cp's OEIS Frontend

A045550 Numbers whose factorial has '8' as its final nonzero digit.

%I A045550 #41 Dec 07 2023 16:44:34
%S A045550 9,10,11,13,15,16,18,22,27,30,31,33,39,44,54,57,70,71,73,77,80,81,83,
%T A045550 89,94,102,105,106,108,114,119,125,126,128,137,142,149,154,157,170,
%U A045550 171,173,184,185,186,188,190,191,193,197,204,207,220,221,223
%N A045550 Numbers whose factorial has '8' as its final nonzero digit.
%C A045550 From _Robert Israel_, Dec 16 2016: (Start)
%C A045550 If n is in the sequence, then:
%C A045550 if n == 0 (mod 5), n+1 is in the sequence;
%C A045550 if n == 1 (mod 5), n+1 is in A045549;
%C A045550 if n == 2 (mod 5), n+1 is in A045548;
%C A045550 if n == 3 (mod 5), n+1 is in A045547. (End)
%H A045550 Robert Israel, <a href="/A045550/b045550.txt">Table of n, a(n) for n = 1..10000</a> (n=1..1000 from Harvey P. Dale)
%H A045550 <a href="/index/Fi#final">Index entries for sequences related to final digits of numbers</a>
%p A045550 count:= 0:
%p A045550 r:= 1:
%p A045550 for n from 2 while count < 100 do
%p A045550   r:= r*n;
%p A045550   if r mod 10 = 0 then r:= r/10^padic:-ordp(r, 5) fi;
%p A045550   if r mod 10 = 8 then count:= count+1; A[count]:= n fi;
%p A045550 od:
%p A045550 seq(A[i], i=1..100); # _Robert Israel_, Dec 16 2016
%t A045550 Select[Range[5,250],Last[Flatten[Most[Split[IntegerDigits[#!]]]]]==8&] (* _Harvey P. Dale_, Jun 11 2014 *)
%t A045550 f[n_] := Mod[6 Times @@ (Rest[ FoldList[{1 + #1[[1]], #2! 2^(#1[[1]] #2)} &, {0, 0}, Reverse[ IntegerDigits[n, 5]]]]), 10][[2]] (* after _Jacob A. Siehler_ & _Greg Dresden_ in A008904 *); f[0] = f[1] = 1; Select[ Range[150], f[#] == 8 &] (* _Robert G. Wilson v_, Dec 28 2016 *)
%o A045550 (Python)
%o A045550 from itertools import count, islice
%o A045550 from functools import reduce
%o A045550 from sympy.ntheory.factor_ import digits
%o A045550 def A045550_gen(startvalue=1): # generator of terms >= startvalue
%o A045550     return filter(lambda n:8==reduce(lambda x,y:x*y%10,(((6,2,4,8,6,2,4,8,2,4,8,6,6,2,4,8,4,8,6,2)[(a<<2)|(i*a&3)] if i*a else (1,1,2,6,4)[a]) for i, a in enumerate(digits(n,5)[-1:0:-1])),6), count(max(startvalue,1)))
%o A045550 A045550_list = list(islice(A045550_gen(),30)) # _Chai Wah Wu_, Dec 07 2023
%Y A045550 Cf. A008904, A045547, A045548, A045549.
%K A045550 nonn,base
%O A045550 1,1
%A A045550 _Jeff Burch_