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A332218
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Numbers k such that A332221(k) = A156552(sigma(k)) is 2*{an odd square}.
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3
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OFFSET
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1,1
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COMMENTS
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Any even term of A332216 must occur also in this sequence.
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LINKS
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EXAMPLE
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a(n) -> sigma(a(n)) -> A156552(sigma(a(n)))
2 = 2^1 * 1^2 -> 3 = 3^1 -> 2 = 2^1 * 1^1,
162 = 2^1 * 3^4 -> 363 = 3^1 * 11^2 -> 98 = 2^1 * 7^2,
441 = 3^2 * 7^2 -> 741 = 3^1 * 13^1 * 19^1 -> 578 = 2^1 * 17^2,
2704 = 2^4 * 13^2 -> 5673 = 3^1 * 31^1 * 61^1 -> 526338 = 2^1 * 3^6 * 19^2,
4225 = 5^2 * 13^2 -> 5673 = 3^1 * 31^1 * 61^1 -> 526338 = 2^1 * 3^6 * 19^2,
and
275194921 = 53^2 * 313^2 -> 281384229 = 3^1 * 7^1 * 181^2 * 409^1 -> 9671406556943421676716050 = 2^1 * 5^2 * 7^2 * 62829235873^2.
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MATHEMATICA
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Select[Range@ 5000, And[IntegerQ[#], OddQ[#]] &@ Sqrt[#/2] &@ Floor@ Total@ Flatten@ MapIndexed[#1 2^(#2 - 1) &, Flatten[Table[2^(PrimePi@ #1 - 1), {#2}] & @@@ FactorInteger@ DivisorSigma[1, #]]] &] (* Michael De Vlieger, Feb 12 2020 *)
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PROG
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(PARI)
istosq(n) = ((1==valuation(n, 2))&&issquare(n/2));
for(n=1, 2^25, if(istosq(A156552(sigma(n*n))), print1(n*n, ", ")); if(istosq(A156552(sigma(2*n*n))), print1(2*n*n, ", ")));
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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STATUS
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approved
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