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A078604 Largest prime factor of the integer formed by truncating the decimal expansion of Pi to n places.

%I A078604 #37 Mar 14 2025 17:10:51
%S A078604 3,31,157,349,103,314159,392699,8263,7853,9786893,28954771,
%T A078604 157079632679,68246533,4304347,67649047,1002742628021,1170899,
%U A078604 990371647,14523877,1186001,1023100457,451661057,1492315939,381315143078063,950007203269
%N A078604 Largest prime factor of the integer formed by truncating the decimal expansion of Pi to n places.
%H A078604 Tyler Busby, <a href="/A078604/b078604.txt">Table of n, a(n) for n = 0..200</a> (terms 0..100 from Ryan Moore, terms 101..169 from Robert G. Wilson)
%H A078604 I. O. Angell, and H. J. Godwin, <a href="http://dx.doi.org/10.1090/S0025-5718-1977-0427213-2">On Truncatable Primes</a> Math. Comput. 31, 265-267, 1977.
%H A078604 Hisanori Mishima, <a href="http://www.asahi-net.or.jp/~KC2H-MSM/mathland/matha1/de_pi000.htm"> Decimal expansions of π (n = 0 to 100)</a>
%H A078604 Hisanori Mishima, <a href="http://www.asahi-net.or.jp/~KC2H-MSM/mathland/matha1/de_pi100.htm"> Decimal expansions of π (n = 101 to 200)</a>
%H A078604 Hisanori Mishima, <a href="http://www.asahi-net.or.jp/~KC2H-MSM/mathland/matha1/de_pi200.htm"> Decimal expansions of π (n = 201 to 250)</a>
%H A078604 <a href="/index/Tri#tprime">Index entries for sequences related to truncatable primes</a>
%F A078604 a(n) = A006530(A011545(n)). - _Michel Marcus_, Dec 28 2013
%F A078604 a(n) = A011545(n) iff n is a term in A060421. - _Robert G. Wilson v_, May 30 2015
%e A078604 a(3) = 157 since 314 = 2*157.
%t A078604 f[n_] := FactorInteger[ IntegerPart[ Pi*10^(n - 1)]][[-1, 1]]; Array[f, 23] (* _Robert G. Wilson v_, May 30 2015 *)
%o A078604 (PARI) a(n) = vecmax(factor(floor(Pi*10^n))[, 1]); \\ _Michel Marcus_, Dec 28 2013
%Y A078604 Cf. A011545, A006530.
%Y A078604 Cf. A089281.
%K A078604 nonn,base
%O A078604 0,1
%A A078604 _Jason Earls_, Dec 09 2002
%E A078604 More terms from _Ryan Moore_, Dec 28 2013