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A331045 a(n) is the least prime number of the form floor(n/10^k) for some k >= 0, or 0 if no such prime number exists.

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%I A331045 #9 Jan 12 2020 13:48:46
%S A331045 0,0,2,3,0,5,0,7,0,0,0,11,0,13,0,0,0,17,0,19,2,2,2,2,2,2,2,2,2,2,3,3,
%T A331045 3,3,3,3,3,3,3,3,0,41,0,43,0,0,0,47,0,0,5,5,5,5,5,5,5,5,5,5,0,61,0,0,
%U A331045 0,0,0,67,0,0,7,7,7,7,7,7,7,7,7,7,0,0,0,83
%N A331045 a(n) is the least prime number of the form floor(n/10^k) for some k >= 0, or 0 if no such prime number exists.
%C A331045 In other words, a(n) is the least prime prefix of n, or 0 if every prefix of n is nonprime.
%C A331045 This sequence is a variant of A331044.
%H A331045 Rémy Sigrist, <a href="/A331045/b331045.txt">Table of n, a(n) for n = 0..10000</a>
%F A331045 a(n) <= n with equality iff n = 0 or n belongs to A069090.
%F A331045 a(n) >= 0 with equality iff n belongs to A202259.
%F A331045 a(n) <= A331044(n).
%e A331045 For n = 23:
%e A331045 - 2 is a prime number,
%e A331045 - hence a(23) = 2.
%o A331045 (PARI) a(n, base=10) = my (d=digits(n, base), p=0); for (k=1, #d, if (isprime(p=base*p+d[k]), return (p))); return (0)
%Y A331045 Cf. A069090, A202259, A331044.
%K A331045 nonn,base
%O A331045 0,3
%A A331045 _Rémy Sigrist_, Jan 08 2020