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A281526
Least number k such that 2*k+n | sigma(k) + sigma(k+n), -1 if such a number is unknown.
1
672, 12, 42, 748, 75, 364, 5, 332, 45, 10, 13, 112, 312, 26, 30, 3604, 3, 3952, 21, 3780, 24, 6, 55906105, 12000, 3192, 44, 1098, 40, 11, 32123069025, 269, 12, 45, 22, 61, 532, 84660, 76, 1044, 70, 13
OFFSET
1,1
COMMENTS
In the first 100 terms, the values of a(42), a(54), a(66) and a(78) are greater than 2.5 * 10^11, if they exist. [Giovanni Resta, Jan 24 2017]
EXAMPLE
For n = 1 -> [sigma(672) + sigma(672+1)] / (672 + 672 + 1) = [2016 + 674] / 1345 = 2690 / 1345 = 2 and 672 is the least number to have this property.
MAPLE
with(numtheory): P:=proc(q) local a; global n, k;
for n from 1 to q do for k from 1 to q do a:=sigma(k)+sigma(k+n);
if type(a/(2*k+n), integer) then print(k); break; fi; if k=q then print(-1); fi; od; od; end: P(10^6);
CROSSREFS
Cf. A000203.
Sequence in context: A172963 A351668 A218795 * A053085 A057695 A233315
KEYWORD
sign,more
AUTHOR
Paolo P. Lava, Jan 23 2017
EXTENSIONS
a(23), a(30), a(49) from Giovanni Resta, Jan 24 2017
STATUS
approved