

A234246


a(n) = {0 < k < n: k*phi(nk) + 1 is a square}, where phi(.) is Euler's totient function.


10



0, 0, 0, 1, 1, 0, 2, 1, 1, 3, 2, 1, 1, 2, 3, 4, 5, 4, 2, 2, 2, 5, 4, 1, 5, 4, 4, 3, 2, 8, 5, 2, 1, 3, 9, 5, 9, 4, 4, 6, 2, 4, 9, 5, 5, 7, 9, 3, 1, 10, 6, 8, 3, 6, 4, 5, 7, 8, 3, 5, 5, 4, 6, 6, 10, 14, 8, 3, 3, 6, 9, 5, 7, 7, 9, 2, 8, 8, 9, 5, 6, 6, 6, 8, 9, 7, 9, 4, 5, 9, 10, 8, 8, 7, 14, 9, 5, 7, 6, 10
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OFFSET

1,7


COMMENTS

Conjecture: (i) a(n) > 0 if n is not a divisor of 6. The only values of n with a(n) = 1 are 4, 5, 8, 9, 12, 13, 24, 33, 49.
(ii) If n >= 60, then k + phi(nk) is a square for some 0 < k < n. If n > 60, then sigma(k) + phi(nk) is a square for some 0 < k < n, where sigma(k) is the sum of all positive divisors of k.
(iii) If n > 7 is not equal to 10 or 20, then phi(k)*phi(nk) + 1 is a square for some 0 < k < n.
(iv) If n > 7 is not equal to 10 or 19, then (phi(k) + phi(nk))/2 is a triangular number for some 0 < k < n.
Note that (n  1)*phi(1) + 1 = n. So a(n) > 0 if n is a square.


LINKS

ZhiWei Sun, Table of n, a(n) for n = 1..10000


EXAMPLE

a(4) = 1 since 3*phi(1) + 1 = 2^2.
a(5) = 1 since 3*phi(2) + 1 = 2^2.
a(8) = 1 since 4*phi(4) + 1 = 3^2.
a(9) = 1 since 8*phi(1) + 1 = 3^2.
a(12) = 1 since 2*phi(10) + 1 = 3^2.
a(13) = 1 since 4*phi(9) + 1 = 5^2.
a(14) = 2 since 2*phi(12) + 1 = 3^2 and 6*phi(8) + 1 = 5^2.
a(24) = 1 since 12*phi(12) + 1 = 7^2.
a(33) = 1 since 3*phi(30) + 1 = 5^2.
a(49) = 1 since 48*phi(1) + 1 = 7^2.


MATHEMATICA

SQ[n_]:=IntegerQ[Sqrt[n]]
a[n_]:=Sum[If[SQ[k*EulerPhi[nk]+1], 1, 0], {k, 1, n1}]
Table[a[n], {n, 1, 100}]


CROSSREFS

Cf. A000010, A000290, A233542, A233544, A233547, A233566, A233567, A233867, A233918, A234200
Sequence in context: A296774 A066099 A254111 * A006375 A327520 A184441
Adjacent sequences: A234243 A234244 A234245 * A234247 A234248 A234249


KEYWORD

nonn


AUTHOR

ZhiWei Sun, Dec 21 2013


STATUS

approved



