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A305947 Number of powers of 7 having exactly n digits '0' (in base 10), conjectured.

%I A305947 #6 Jun 23 2018 09:19:19
%S A305947 10,11,12,13,9,10,9,7,10,14,21,10,18,7,11,11,12,15,17,10,11,6,10,16,
%T A305947 13,9,7,9,11,12,10,16,7,16,9,14,13,13,9,17,14,12,11,9,13,9,12,12,9,12,
%U A305947 14
%N A305947 Number of powers of 7 having exactly n digits '0' (in base 10), conjectured.
%C A305947 a(0) = 10 is the number of terms in A030703 and in A195908, which includes the power 7^0 = 1.
%C A305947 These are the row lengths of A305927. It remains an open problem to provide a proof that these rows are complete (as for all terms of A020665), but the search has been pushed to many orders of magnitude beyond the largest known term, and the probability of finding an additional term is vanishing, cf. Khovanova link.
%H A305947 M. F. Hasler, <a href="/wiki/Zeroless_Powers">Zeroless powers</a>, OEIS Wiki, March 2014, updated 2018.
%H A305947 T. Khovanova, <a href="https://blog.tanyakhovanova.com/2011/02/86-conjecture/">The 86-conjecture</a>, Tanya Khovanova's Math Blog, Feb. 2011.
%H A305947 W. Schneider, <a href="http://web.archive.org/web/20050407120908/http://www.wschnei.de:80/digit-related-numbers/nozeros.html">No Zeros</a>, 2000, updated 2003. (On web.archive.org--see A007496 for a cached copy.)
%o A305947 (PARI) A305947(n,M=99*n+199)=sum(k=0,M,#select(d->!d,digits(7^k))==n)
%o A305947 (PARI) A305947_vec(nMax,M=99*nMax+199,a=vector(nMax+=2))={for(k=0,M,a[min(1+#select(d->!d,digits(7^k)),nMax)]++);a[^-1]}
%Y A305947 Cf. A030703 (= row 0 of A305927): k such that 7^k has no 0's; A195908: these powers 7^k.
%Y A305947 Cf. A020665: largest k such that n^k has no '0's.
%Y A305947 Cf. A063606 (= column 1 of A305927): least k such that 7^k has n digits '0' in base 10.
%Y A305947 Cf. A305942 (analog for 2^k), ..., A305946, A305938, A305939 (analog for 9^k).
%K A305947 nonn,base
%O A305947 0,1
%A A305947 _M. F. Hasler_, Jun 22 2018