1,3

The sum of digits of k! is approximately (9/2)*(d-z), where d=A034886(k) is the number of digits of k!, which is about (log(k/E)*k + log(2*k*Pi)/2)/log(10), and z=A027868(k) is the number of trailing zeros of k!, which is Sum_{j>=1} floor(k/5^j). - Giovanni Resta, Aug 28 2013

a(9) > 2.235*10^9. - Hans Havermann, May 16 2014

k! has ~ k log_10(k) digits, so its digit sum is typically close to C*k*log_10(k) for some constant C. A random number around j has probability something like log(j)/(j log log(j)) of being a factorial, so the probability that the digit sum of k! is a factorial should be something like const/(k log log k). The sum of this diverges, so we should expect infinitely many terms in the sequence. - Robert Israel, Aug 08 2014

Shyam Sunder Gupta, Fascinating Factorials, Exploring the Beauty of Fascinating Numbers, Springer (2025) Ch. 16, 411-442.

"Mouhaha" Digit sums and factorials

The sum of the digits of 21966! is 362880 = 9!.

The sum of the digits of 176755! is 3628800 = 10!.

The sum of the digits of 182624820! is 6227020800 = 13!.

lst = {0}; k = p = 1; fctl = Range@ 15!; While[k < 180000, p = p*k; While[ Mod[p, 10] == 0, p /= 10]; If[ MemberQ[ fctl, Plus @@ IntegerDigits@ p], Print[k]; AppendTo[lst, k]]; k++]; lst (* Robert G. Wilson v, Feb 18 2014 *)

With[{fcts=Range[20]!}, Select[Range[0, 22000], MemberQ[fcts, Total[IntegerDigits[#!]]]&]] (* Harvey P. Dale, Jan 06 2024 *)

(PARI) lpf(n)=my(f=factor(n)[, 1]); f[1]

factorial_lval(n, p)={

my(s);

while(n\=p, s+=n);

s

};

isfactorial(n)={

if(n<3, return(n>0));

my(v2=valuation(n, 2), mn=v2+1, mx=mn+log(v2+1.5)\log(2), t, c);

while (mx - mn > 1,

t = mn + (mx - mn)\2;

c = factorial_lval(t, 2);

if (c < v2,

mn = t+1

,

if (c > v2,

mx = t-1

,

mx = bitor(t, 1);

mn = max(mn, mx-1)

)

)

);

if (mn < mx,

my(p=lpf(mx));

t = valuation(n, p);

c = factorial_lval(mx, p);

if (t > c, return(0));

if (t == c,

mn = mx

)

);

n==mn!

};

is(n)=isfactorial(sumdigits(n!))

Cf. A229024, A004152.

Sequence in context: A062929 A038105 A143716 * A258107 A307256 A107656

Adjacent sequences: A228308 A228309 A228310 * A228312 A228313 A228314

nonn,base,hard,nice,more

Charles R Greathouse IV, Aug 27 2013

a(8) from Hans Havermann, Mar 24 2014

approved