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A303447
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Numbers m such that both m and 2m+1 are Fermat pseudoprimes base 2 (A001567).
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5
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OFFSET
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1,1
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COMMENTS
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No other terms below 2^64.
Terms a(2) and a(3) are of the form (4^k-1)/3=A002450(k). Terms A002450(k) belong to this sequence for k in A303009. Correspondingly, a(4) <= A002450(A303009(3)) = (4^41-1)/3 = 1611901092819505566274901.
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LINKS
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CROSSREFS
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Numbers 2*a(n)+1 are listed in A303448.
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KEYWORD
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bref,hard,more,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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