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A283802 Numbers k such that the concatenation of the first k odd composite numbers is a prime. 1
21, 73, 132 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Indices k for which A283801(k) is prime.

A283801(21) = 91521252733353945495155576365697577818587 is a 41-digit prime; A283801(73) = 91521...247249253 is a 193-digit prime; A283801(132) = 91521...423425427 is a 370-digit prime.

Next term, if it exists, will be more than 5028.

LINKS

Table of n, a(n) for n=1..3.

Eric Weisstein's World of Mathematics, Consecutive Number Sequences

MAPLE

P:=proc(q) local a, k, n; a:=0; k:=0; for n from 9 by 2 to q do

if not isprime(n) then k:=k+1; a:=a*10^(ilog10(n)+1)+n;

if isprime(a) then print(k); fi; fi; od; end: P(10^3); # Paolo P. Lava, Mar 17 2017

MATHEMATICA

k = 2; cc = oc = 0; lst = {}; While[k < 428, If[OddQ@k && !PrimeQ@k, cc = cc*10^IntegerLength@k + k; oc++; If[PrimeQ[cc], AppendTo[lst, oc]]; k++]; lst  (* Robert G. Wilson v, Mar 18 2017 *)

CROSSREFS

Cf. A007908, A019518, A046035, A069151, A071904, A283801.

Sequence in context: A160435 A240824 A109357 * A082041 A316887 A296155

Adjacent sequences:  A283799 A283800 A283801 * A283803 A283804 A283805

KEYWORD

base,nonn,bref,more

AUTHOR

XU Pingya, Mar 17 2017

STATUS

approved

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Last modified October 22 10:29 EDT 2021. Contains 348160 sequences. (Running on oeis4.)