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A280427 a(n) is a prime, such that a(n) = p^d-2 where p is a prime and d is the number of digits of p. 1

%I #29 Mar 03 2024 10:15:03

%S 3,5,167,359,839,1367,1847,2207,3719,5039,7919,1295027,3442949,

%T 9393929,13997519,21253931,49430861,84604517,95443991,237176657,

%U 329939369,384240581,487443401,633839777,904231061,1078193566319,1427186233199,1556727840719,1985193642959

%N a(n) is a prime, such that a(n) = p^d-2 where p is a prime and d is the number of digits of p.

%C These numbers (proved for all p < 500) are a subset of A007528. For all even p, such numbers are a subset of A007528. The sequence is a subset of all numbers f(i) such that f(i) = i^d-2 (d - number of digits of integer i) and f(i) is a prime: e.g., f(15) is prime while f(15) = 15^2-2 = 223.

%F a(n) = p^d-2, a(n) is prime, p is a prime and d is the number of digits of p.

%e If p=5, then d=1 and a(1)=3; if p=7, then d=1 and a(2)=5; if p=13, then d=2 and a(3)=167; etc.

%t Select[Array[#^IntegerLength@ # - 2 &@ Prime@ # &, 200], PrimeQ] (* _Michael De Vlieger_, Jan 03 2017 *)

%Y Cf. A007528.

%K nonn,base

%O 1,1

%A _Sergey Pavlov_, Jan 02 2017

%E More terms from _Michael De Vlieger_, Jan 03 2017

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Last modified March 28 05:39 EDT 2024. Contains 371235 sequences. (Running on oeis4.)