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 A249806 Smallest odd number k>1 such that k*2^prime(n)-1 is also prime. 1
 3, 3, 7, 3, 3, 9, 7, 51, 13, 7, 15, 21, 15, 3, 31, 147, 45, 69, 43, 73, 15, 69, 91, 19, 51, 81, 3, 25, 9, 85, 103, 55, 169, 225, 109, 145, 15, 103, 615, 69, 259, 69, 63, 45, 285, 471, 9, 255, 169, 489, 69, 273, 427, 43, 391, 169, 201, 21 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If prime(n) is a Mersenne prime exponent then 2^prime(n)-1 is a prime < k*2^prime(n)-1. LINKS Pierre CAMI, Table of n, a(n) for n = 1..1000 MAPLE 3*2^2-1=11 prime so a(1)=3. 3*2^3-1=23 prime so a(2)=3. 3*2^5-1=95 composite, 5*2^5-1=159 composite, 7*2^5-1=223 prime so a(3)=7. MATHEMATICA a249806[n_Integer] := Catch[Module[{k}, For[k = 3, k < 10^5, k += 2, If[PrimeQ[k*2^Prime[n] - 1], Throw[k], 0]]]]; a249806 /@ Range[120] (* Michael De Vlieger, Nov 11 2014 *) PROG (PFGW & SCRIPT) SCRIPT DIM j, 0 DIM k DIM n DIMS t OPENFILEOUT myf, a(n).txt LABEL loop1 SET j, j+1 IF j>1000 THEN END SET k, p(j) SET n, 1 LABEL loop2 SET n, n+2 SETS t, %d, %d, %d\,; j; k; n PRP n*2^k-1, t IF ISPRP THEN GOTO a GOTO loop2 LABEL a WRITE myf, t GOTO loop1 (PARI) s=[]; forprime(p=2, 500, k=3; q=2^p; while(!ispseudoprime(k*q-1), k+=2); s=concat(s, k)); s \\ Colin Barker, Nov 06 2014 CROSSREFS Cf. A135434. Sequence in context: A076560 A359048 A096915 * A249382 A317929 A285387 Adjacent sequences: A249803 A249804 A249805 * A249807 A249808 A249809 KEYWORD nonn AUTHOR Pierre CAMI, Nov 06 2014 STATUS approved

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Last modified September 18 13:18 EDT 2024. Contains 376000 sequences. (Running on oeis4.)