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A244447 a(n) is the smallest integer m such that m-n is composite and phi(m+n) + sigma(m-n) = 2*m. 5
11, 8, 13, 37350, 25, 18, 28, 20, 61, 22, 44, 40, 52, 250, 39, 60, 68, 60, 58, 76, 168, 46, 92, 69, 2040, 56, 126, 84, 114, 140, 88, 74, 108, 90, 288, 92, 148, 108, 283, 324, 164, 180, 100, 40878, 125, 474, 162, 108, 773, 71, 111, 240, 168, 315, 148, 194, 564, 390, 128, 144, 124, 164, 153, 279, 1008, 162, 102, 152, 432, 222 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For each n, a(n)>n and like a(n)-n, a(n)+n is also composite.

If both numbers p and p+2n are primes then x=p+n is a solution to the equation phi(x+n)+sigma(x-n)=2x. But for these many solutions x, both numbers x-n and x+n are primes.

LINKS

Table of n, a(n) for n=1..70.

EXAMPLE

a(1)=11 because 11-1 is composite, phi(11+1)+sigma(11-1)=2*11 and there is no such number less than 11.

MATHEMATICA

a[n_]:=(For[m=n+1, PrimeQ[m-n]||EulerPhi[m+n]+DivisorSigma[1, m-n]!=2m, m++]; m); Table[a[n], {n, 70}]

PROG

(PARI)

a(n)=m=n+4; while(isprime(m-n)||eulerphi(m+n)+sigma(m-n)!=2*m, m++); m

vector(100, n, a(n)) \\ Derek Orr, Aug 30 2014

CROSSREFS

Cf. A000010, A000203, A244446, A244448.

Sequence in context: A164059 A306494 A068974 * A206420 A304699 A133236

Adjacent sequences:  A244444 A244445 A244446 * A244448 A244449 A244450

KEYWORD

nonn

AUTHOR

Jahangeer Kholdi and Farideh Firoozbakht, Aug 30 2014

STATUS

approved

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Last modified March 21 20:25 EDT 2019. Contains 321382 sequences. (Running on oeis4.)