42717

Appears in sequences

• Numbers k such that sigma(phi(k)) - phi(sigma(k)) is nonzero and divisible by sigma(k), that is A065395(k)/A000203(k) is a nonzero integer.at n=31A092588
• Expansion of (1+x)c(x^2)/((1-xc(x^2))*sqrt(1-4x^2)), c(x) the g.f. of A000108.at n=15A117186
• Odd numbers n divisible by 3 such that for all k >= 1 the numbers n*2^k + 1 and n*2^(k+1) + 1 do not form a pair of primes.at n=1A243857
• Place two n-gons with radii 1 and 2 concentrically, forming an annular area between them. Connect all the vertices with line segments that lie entirely within that area. Then a(n) is the number of edges in that figure.at n=26A337702