A217768 -id:A217768 - cp's OEIS frontend

A217704 Smallest number k > n for which sigma(k-n) = sigma(k+n).

34, 53, 23, 19, 26, 41, 31, 38, 49, 52, 68, 82, 112, 80, 103, 76, 110, 123, 166, 83, 101, 136, 219, 164, 130, 186, 147, 133, 381, 254, 208, 149, 253, 169, 131, 246, 172, 332, 404, 166, 245, 312, 148, 209, 309, 241, 487, 328, 217, 260

Offset: 1

Jayanta Basu, Mar 26 2013

nonn

The sigma() in the definition is the sum-of-divisors function A000203.

			a(26)=186 because sigma(186-26)=sigma(160)=378=sigma(212)=sigma(186+26) and there is no k such that 26<k<186 for which sigma(k-26)=sigma(k+26).

Andrew Howroyd, Table of n, a(n) for n = 1..10000

Cf. A217768.

Table[Min[Select[Range[500],DivisorSigma[1, # - n] == DivisorSigma[1, # + n] && # > n &]], {n,50}]

a(n)={my(k=n+1); while(sigma(k-n)<>sigma(k+n), k++); k} \\ Andrew Howroyd, Feb 25 2018

A218466 Least k > n for which phi(k - n) = phi(k + n) or 0 if no such k exists.

Original entry on oeis.org

5, 10, 27, 17, 25, 54, 23, 34, 61, 47, 55, 108, 47, 46, 139, 68, 58, 122, 71, 85, 144, 95, 115, 207, 101, 94, 183, 92, 145, 278, 104, 136, 177, 116, 175, 244, 161, 142, 306, 149, 184, 283, 191, 187, 410, 230, 235, 267, 146, 202, 299, 188, 157, 366, 275, 184

Offset: 1

Irina Gerasimova, Mar 26 2013

nonn

Is there an upper bound for a(n) for a given n? - Michael B. Porter, Apr 06 2013

			a(3) = 27 since phi(27 - 3) = phi(24) = 8 and phi(27 + 3) = phi(30) = 8, and 27 is the smallest number greater than 3 for which the two are equal.

Donovan Johnson, Table of n, a(n) for n = 1..1000

Cf. A000010, A217768, A223091.

/* will not terminate if k does not exist */
a218446(n) = {local(k); k = n + 1; while(eulerphi(k - n) <> eulerphi(k + n), k = k + 1); k} \\ Michael B. Porter, Mar 30 2013

Extended by R. J. Mathar, Mar 27 2013

cp's OEIS Frontend

A217704 Smallest number k > n for which sigma(k-n) = sigma(k+n).

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