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A340870
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a(n) is the smallest prime p such that p - 1 has 2*n divisors.
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0
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3, 7, 13, 31, 113, 61, 193, 211, 181, 241, 13313, 421, 12289, 2113, 1009, 1321, 2424833, 1801, 786433, 2161, 4801, 15361, 155189249, 2521, 6481, 61441, 6301, 8641, 3489660929, 12241, 3221225473, 7561, 64513, 1376257, 58321, 12601, 206158430209, 8650753, 184321
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OFFSET
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1,1
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COMMENTS
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LINKS
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FORMULA
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tau(a(n) - 1) = 2*n.
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EXAMPLE
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a(4) = 31 because 31 is the smallest prime p such that p - 1 has 2*4 divisors; tau(30) = 8.
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MATHEMATICA
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a={}; For[n=1, n<=40, n++, i=1; While[DivisorSigma[0, Prime[i]-1]!=2n, i++]; AppendTo[a, Prime[i]]]; a (* Stefano Spezia, Jan 25 2021 *)
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PROG
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(Magma) Ax:=func<n|exists(r){m:m in[2..10^7] | IsPrime(m) and #Divisors(m - 1) eq n*#Divisors(m)}select r else 0>; [Ax(n): n in[1..20]]
(PARI) a(n) = my(p=2); while(numdiv(p-1) != 2*n, p=nextprime(p+1)); p; \\ Michel Marcus, Jan 25 2021
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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