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 A215689 Smallest prime whose decimal expansion consists of the concatenation of a 1-digit square, a 2-digit square, a 3-digit square, ..., and an n-digit square, or 0 if there is no such prime. 8

%I

%S 0,149,125441,1161002209,116100102414161,116100102410000106929,

%T 1161001024100001004891442401,116100102410000100489100000010169721,

%U 116100102410000100489100000010004569100460529,1161001024100001004891000000100045691000000001009269361

%N Smallest prime whose decimal expansion consists of the concatenation of a 1-digit square, a 2-digit square, a 3-digit square, ..., and an n-digit square, or 0 if there is no such prime.

%C The n-th term has n(n+1)/2 digits (cf. A000217). There are (0, 3, 29, 991, 175210, ...) primes of that form, for n = 1, 2, 3, .... We can conjecture that a(n) > 0 for all n, and even that the terms converge to the concatenation of (s(1), s(2), s(3), ...) where s(n) is the smallest n-digit square, cf. formula. - _M. F. Hasler_, Dec 31 2020

%H M. F. Hasler, <a href="/A215689/b215689.txt">Table of n, a(n) for n = 1..44</a> (all terms < 10^1000), Dec 31 2020.

%F a(n) ~ 10^(n(n+1)/2) * 0.1161001024100001004891000000100045691... - _M. F. Hasler_, Dec 31 2020

%e a(2) = 149, which is a prime, and the concatenation of 1 = 1^2 with 49 = 7^2.

%e a(3) = 125441, which is a prime, and the 1 = 1^2 with 25 = 5^2 with 441 = 21^2.

%o (PARI) apply( {A215689(n)=forvec(v=vector(n, k, [ceil(10^((k-1)/2)), sqrtint(10^k-1)]), ispseudoprime(n=eval(concat([Str(k^2)|k<-v])))&&return(n))}, [1..11]) \\ _M. F. Hasler_, Dec 31 2020

%Y Cf. A000040, A000290, A000217, A215641, A215647.

%Y Cf. A215692 (analog for cubes).

%K nonn,base

%O 1,2

%A _Jonathan Vos Post_, Aug 20 2012

%E More terms (up to a(10)) from _Alois P. Heinz_, Aug 21 2012

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Last modified May 18 22:46 EDT 2021. Contains 344006 sequences. (Running on oeis4.)