Research Article Spatial Approach of Artificial Neural Network for Solar Radiation Forecasting: Modeling Issues

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1 Jourl of Solr Eergy Volume 2015, Article ID , 13 pges Reserch Article Sptil Approch of Artificil Neurl Network for Solr Rditio Forecstig: Modelig Issues Yshwt Kshyp, 1 Akit Bsl, 2 dailk.so 3 1 School of Egieerig, Idi Istitute of Techology Mdi (IIT Mdi), Room No. 106, Mdi Cmpus, Mdi , Idi 2 Mechicl Egieerig Deprtmet, Idi Istitute of Techology Roorkee (IITR), Roorkee , Idi 3 School of Computig d Electricl Egieerig, Idi Istitute of Techology Mdi (IIT Mdi), Mdi , Idi Correspodece should be ddressed to Yshwt Kshyp; yshwt.kshyp@gmil.com Received 25 September 2014; Revised 5 December 2014; Accepted 18 December 2014 Acdemic Editor: Jysuder M. S. Bdr Copyright 2015 Yshwt Kshyp et l. This is ope ccess rticle distributed uder the Cretive Commos Attributio Licese, which permits urestricted use, distributio, d reproductio i y medium, provided the origil work is properly cited. Desig of eurl etworks rchitecture hs bee doe o settig up the umber of euros, delys, d ctivtio fuctios. The expected model ws iitited d tested with Idi solr horizotl irrditio (GHI) metrologicl dt. The resultsre ssessed usig the effect of differet sttisticl errors. The effort is mde to verify simultio cpbility of ANN rchitecture ccurtely, o hourly rditio dt. ANN model is well-orgized techique to estimte the rditio usig differet meteorologicl dtbse. I this pper, we hve used ie sptil eighbour loctios d 10 yers of dt for ssessmet of eurl etwork. Hece, overll 90 differet iputs re compred, o customized ANN model. Results show the flexibility with respect to sptil oriettio of model iputs. 1. Itroductio Utility workig i the field of solr eergy productio is compulsory to develop its forecst bility i diverse climtic situtio [1, 2]. Uusul fluctutios occur i direct d diffuse icidet irrditio (up to 100%), due to existece of clouds [3, 4]. I recet yers, rtificil eurl etworks (ANN) hve bee used for forecstig d regressio of solr rditio i differet ltitudes d climte coditios. Sice its developmet, o ccurte method hs bee foud to solve stbility d ucertity of delyed etworks. Therefore, delyed etwork stbility study hs gi dditiol importce. I recet yers, wreess of delyed eurl etworks hs icresed. It is ecessry for the ANN to pper i globlly stble regio. A umbers of method hve bee developed to determie optimum umerl of delys, especilly, by mes of cross vlidtio [5 11]. Numerous reserchers hve mde efforts d suggested techiques for settig umber of euro i rtificil eurl etwork. I 2011, Gozlez-Crrsco et l. cosidered ew method of optimizig the umber of euros i dt miig. I 1998, Fujit documeted umericl pproximtio of the umber of euros of the eurl etwork. I 1997, Tmur d Tteishi observed tht i stdrd Akike recovered euro produced idepedetly. I 2003, Zhg et l. develop suitble methods for euros. I 1995 Li et l. observed volutry pproch euros of the time series predictio [12 16]. Most importt works hs bee doe towrds usig of ctivtio fuctios. Jord obti the logistic fuctio which is stdrd desig of the ext prospect i biry clssifictio covolutio [17]. Yo d Liu ehced the structure of complete etworks with two strge ctivtio fuctio, those re sigmoid d Gussi bsis [18]. Sope et l. vilble umber of test pperce o multilyer feed forwrd etworks used sie ctivtio fuctio [19, 20]. However, the difficulty log with trsfer fuctios is ot theoreticl coditio for their selectio [21]. The literture ssessmet demostrtes tht the ANNs hve ot bee used for sptil domi lysis. I this pper, cosider such specil chrcteristics of sptil domi with solr rditio time series dt. Associted study cosidered source of 3 3sptil mtrixes d 10 yers of time series. Sptiotemporl spect hs bee used for ANN lysis. Results

2 2 Jourl of Solr Eergy re evluted with stdrd sttistics errors. All sptil iputs re tested bsed o dely, euros, d ctivtio fuctios s modelig fctor. 2. Dt Processig 2.1. Dt Collectio. The defult vlue of the rditio re is bout 1360 Wm 2 d crries diffuse horizotl rditio (GHI) d direct orml irrditio (DNI). These two terms re used to clculte the globl horizotl irrditio (GHI) s follows: GHI = DHI + DNI cos (Φ), (1) where Φ is the solr zeith gle [22]. Logitudil dt for Idi i the form of 10 kilometers ( resolutio) poits i spce [23] re vilble. Suy stellites re used every hour from Jury 2003 to Jue This work icludes the ltitude d logitude d ccumulted i GHI i orther Idi, s show i Figure 1. Thedtreirectgulrgrid km 2 plced.theghiexistsiprticulrtimezoe5.5to cetrl loctio of the proposed re i Figure 2 for 5,000 hours of Dt Processig. Importt pproch of dt miig is used to scle the iput d trget i the ANN. Thus, the ormliztio is used with the stdrd devitio d the me of the triig dt set. Therefore, for the dt of solr rditio with zero me d stdrd devitio uit of the ext equtio: y=(x x me ) ( y std )+y x me, (2) std where y, y me, y std, x, x me,dx std dt sets re dt, me, dsldereddevitiooftrgetddtdmed sldered devitio of triig set, respectively. The dt sets re i form of time series d require iterpretig ito sptil domi. The iput dt sets re prepred primrily i physicl positio lloctig to topology fuctio. Usig rectgulr grid fuctio my lso be used similr to hexgo or rdom topology. It begis with iput dt i rectgulr grid similr to tht show i Figure 3 for istce. Assume tht sptil dt re i 3 3rrys of ie differet loctios. The iput 1 hs positio (1, 1), iput 2 hs the positio (1, 2), iput 3 hs the positios (1, 3) d (2, 1), d so forth. Aother three-dimesiol topology of sptil dt set is show i Figure 4. Theceter iput hs eighbourhoods of icresig dimeter erby it. A eighbourhood of dimeter 1 icludes the ceter d its istt eighbours. The eighbourhood of dimeter 2 cosists of the dimeter 1 d its immedite eighbours. The rectgulr topology fuctio d ll the eighbourhoods for multiple iput mp re chrcterized by M-by-M mtrix of distce. 3. Artificil Neurl Network (ANN) 3.1. Neurl Networks for Hourly Solr Rditio. Artificil eurl etworks processig systems hve bility to ler 74.65/ Ltitude W, 180 N, / E, S, / / Logitude Figure 1: Sptil distributio of solr rditio d dt poits. GHI (W/m 2 ) Solr rditio (GHI) Time (hours) Figure 2: Globl horizotl irrditio plot (GHI) of 5000 hours. through iformtio [24]. Figure 2 shows clssic demostrtio for euro rchitecture where I 1,I 2,I 3,...,I m, w 1,w 2,w 3,...,w m, b, X, O,df( ) re the sigls, weights, bis, ctivtio potetil, output sigl, d ctivtio fuctio, respectively. Afterwrds, oe c supervise tht the euro efficiecy is give by O =f{b + m i=1 w i X i }. (3) Such usul etwork rchitecture is usully referred to s multilyer eurl etwork [25].It is bsed o its topology d the mout of the weights i the iput lyer. The simplifictio of rtificil eurl etwork is the cpcity of replictig preferred sigls for differet iput sigls d the cpcity of holdig the dymics of the system [26]. However, to determie umber of euros i ech lyer is ot trivil. Reserch rticles hve explied cses where uderfittig d overfittig might occur whe smller d lrger umbers of euros re used i the etwork [16].

3 Jourl of Solr Eergy 3 Two-dimesio sptil model Hidde lyer Output lyer Rows Home euro Neighborhood Iput d w b d w + + b Output Neighborhood Figure 6: Typicl custom euro rchitecture. Colums Figure 3: Sptil pproch of Neurl Network distributio i 2D of Solr Rditio km prt Neighbor iputs 2 5 3D sptil model Neighborhood 1 Neighborhood 1 Neighborhood 1 Neighborhood 1 Neighborhood 1 Neighborhood 1 Neighborhood 1 Neighborhood 1 Trget dt set Hours of dt Figure 4: Sptil pproch of Neurl Network distributio i 3D of Solr Rditio. Numerous pproches hve bee used without success to fid the pproprite method for computig the umber of euros i ech lyer [27]. However, due to their simplicity, these methods hve bee extesively utilized i time series lysis [28]. The ultimte outcome of ANN is group of weights d iput vribles for lier d olier process [29, 30]. I this pper, we hve used oe-output odes i the outer lyer for forecstig of GHI. The ccurcy of the vrious ANNmodelsiscompredtothemostcorrectmodelofhourly solr rditio, s described below. Custom Network. Strt etwork desigig usig toolbox offer specilchoice.tocostructcustomrrgemets,strtwith empty etwork d set its properties s specil s show i Figure 6. The etwork used umerous fuctio properties tht hve bee set i my wys, s desired for etwork rchitecture. I this sectio use the slight orml etwork d differet sptil iputs. The iput etwork recogizes ormlized vlue rge from to +1 of rditio.the umber of lyers used for this etwork is two, iitilized with the Nguye Widrow lyer iitiliztio method d tried with the Leveberg-Mrqurdt s described below. To tri greter umber of lyers it eeds dditiol time. While there re simply 2 lyers i solr rditio etwork for defult custom models tht c be tried for 1000 epochs, the literture described i terms of ccurcy, whe usig 2 lyers i ANN, is much higher th tht of higher lyers. Hece, we c cosider the etwork with 2 hidde lyers, which provides the highest precisio vlue, s the most pproprite etwork for this problem. Ay output vectors i output lyer will ler to ssocite the coected trget vectors with miiml me squred error icludig weights d bises [29 32]. i 1 b 1 W 1 i 2 W 2 Σ x f(x ) O W m i m Figure 5: Bsic euro etwork rchitecture Neurl Network Triig. The LMA deliver ehced performcessoosbeigcoectedwithclssicbck propgtio processes. Due to Newto s techique the etworks moderized lw is w +1 =w H g, (4) w +1 =w [J T J + ε I] g, where w,, H, g, J, I, dε re the etwork weight mtrix, umber of repetitios, the Hessi mtrix, the grdiet mtrix, the Jcobi mtrix, the idetity mtrix, d sclr, respectively [25]. The projected etworks were tried by 70% of the delivered dt, wheres the cotiuig 15% ws used for

4 4 Jourl of Solr Eergy vlidtio d remiig 15% ws for testig the tried etwork. Therefter, the tried etworks were used for forecst usig lst 100 dys of dt. The suggested ANNs forecst the solr rditio t ceter poit of positio 5 from Figure 3 (i terms of 2 2mtrix) from 2012 yer of dt, dtthttimetheforecsteffectswereequtedthrough themesureddt.idividullyiputsof9sptilloctios d10yerofdt(totl90)wereverifiedseprtelybythe hourly rditio vlues, d ll of the suggested iputs were mtched collectively by the RMSE vlues of solr rditio. These specil etworks were compred with triig RMSE error. 4. Model Evlutio Criteri Filly, the model hs bee selected bsed o the lowest forecstig error. The estimtio of error c be my forms such s root me squre error (RMSE) d MAE (me bsolute error): MAE = 1 I t I t, t=1 RMSE = 1 (I t I t ) 2. I (5) there re differet expressios for error estimtio where I t represets mesured vlue t forecsted horizo t d I t is forecsted vlue. Also represets the totl umber of test smples. This vlidtio process defies the model ccurcy d stop itertio process for ANN model [33]. 4.1.ModelBehviorwithDelys. I this sectio, we used oe iput t time with dely out of 90 differet rditio dt. The hidde lyer required ws ofte (10) te euros; the iput dely vries from oe to thirty (30). For GHI, the wrog lrm frequecy ws costtly low while beig tried o the mximum mout of pst dt [34]. All cofigurtio models re tested for 30 times t cotiuous time dely of 1 to 30. The etwork hs to optimize t miimum root me squre (RMSE) o the triig dt set [28]. Figure 7 d Tble 1 illustrte evlutio of mesured dforecstedvluesbythesuggestedanns;thisevlutio ws built o hourly verges of globl rditios. Estblished rditio o iput umbers (from the Tble 1) (IN-10, IN- 3, d IN-9) hd very smll dissimilrity o globl forecst. IN-10 ws superior d the IN-3 i terms of rditio forecstig, but both hd comprble vlues. IN-90 ws the poorest betwee the suggested etworks with the highest error i globl rditio forecstig. It strts error with 32.23% RMSE t 5 delys d eds with 19.63% t 23 delys d further remis costt i betwee for ll other iputs d delys. Tble 1 demostrted summry evlutio of the suggested models by percetge performce of the MAE, MSE, d RMSE. These results show tht the solr rditio forecstig differs too much irrespective of ANN iputs. Forecstig is ffected bsed o umber of time delys t=1 (5) (W/m 2 ) Solr rditio plot: forecsted versus ctul outputs differet forecsted plots Actul plot Time (hours) Figure 7: Rditio forecstig; sptil compriso of ANN models durig triig with time dely. GHI (W/m 2 ) Solr rditio plot: forecsted versus ctul Time (hours) differet iputs Figure 8: Rditio forecstig; sptil compriso of ANN models durig triig with time dely d euros. pplied i differet ANN iputs. The dely decides the covergece property of ANN iputs. Tble 1 shows tht some of the iput coverges very fst with miimum error s IN-51 with 0 time delys d roud percet of testig RMSE error. Similrly other comprble methods of IN-16 show the percet of testig RMSE error but log time dely (29). The hourly correltio fctor betwee two cler skies dys is lmost oe dy, which is perfect for time series predictio cosiderig the sme itervl. I terms of sptil lysis top best five testig results show percetge i RMSE 19.63, 20.07, 20.17, 20.66, d of eighbour positios 11(1), 13(7), 33(9), 12(4), d 21(2) d delys of 23, 27, 26, 7, d 8. O the other hd, compre the worse results re 29.22, 29.63, 30.57, d of positio 22(5),

5 Jourl of Solr Eergy 5 Tble 1: Rditio forecstig; sptil compriso of ANN models with time dely. Number of iputs Triig Forecstig MAE RMSE MAE RMSE Yer Dely Colum Row Rkig Iput rditio GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI

6 6 Jourl of Solr Eergy Tble 1: Cotiued. Number of iputs Triig Forecstig MAE RMSE MAE RMSE Yer Dely Colum Row Rkig Iput rditio GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI GHI Strs ( ) shows best three results i order. 31(3), 22(5), 32(6) d 33(9) with dely of 23, 0, 6, 6, d 5, respectively. From these results there is o specil ptter of sptilpositio;thismybeduetodifferetdelycorrelte betwee iputs d outputs. But from results, positio 5 ppers i worst cse s compred to the best performce from eighbour plces. O the other hd, temporl lysis shows cler ptter betwee iput d output with respect to differet yers of dt. I the tble, top best results re from 2011, 2012, 2012, 2011, d 2010, respectively. If compred with theworseresultswhichre2005,2003,2003,2003,d2003, respectively, it clerly shows tht the best result performed by ANN model depeds o closer yer of dt. The cotributio

7 Jourl of Solr Eergy 7 Number of iputs Tble 2: Rditio forecstig; sptil compriso of ANN models with time dely d euros. %performce (MAE) %triig %forecstig Triig Vlidtio Testig MSE RMSE MSE RMSE Yer Dely Neuros Colum Row Rkig

8 8 Jourl of Solr Eergy Number of iputs Tble 2: Cotiued. %performce (MAE) %triig %forecstig Triig Vlidtio Testig MSE RMSE MSE RMSE Yer Dely Neuros Colum Row Rkig

9 Jourl of Solr Eergy =pureli () = hrdlims () = stli () =posli () = hrdlim () = compet () =stlis () =logsig () = tsig () Figure 9: ANN models used with differet trsfer fuctio. of dely is rdom, some iputs perform well with higher dely d some perform t lower dely, it ll depeds o property of dt with respect to time Model Behvior with Neuros. Tble 2 demostrtes tht the projected euro model offers superior results for evlutio.thecurrettechiqueisusedtocotrolthe umber of euros bsed o tril-d-error. It strts with miimum umber of euros d icresed euro to its mximum limit. The drwbck is tht it is time cosumig d there is o surety of settig the euro. The prticulr mesures for 90 iputs used rge of euros t itervl of 10 euros d trget for miimized MAE vlue. Sice the smllest RMSE demostrtes the estimtio method ccurcy t locl level or smll umber of dt sets, MAE idictes globl ccurcy. I this cse previous obtied outcome delys re used d stdrd trsfer fuctios, hyperbolic tget sigmoid (tsig), for respective umbers of iputs, re used s well. The ccurcy ws tested by usig the triig sets results i ech cse. Figure 8 d Tble 2 show the results with ll errors, performce coefficiets, delys, d differet euros for ech iput. It is preferred tht simplifictio cpbility icresed while the umber of euros is icresed. I solr rditio estimtio problem, the precisio degree of the productivity ws 3.61, 4.63, d 3.98% i triig, vlidtio, d testig, respectively, while 190 euros re used i IN-3 t dely (27). Comprtively the sme result ws obtied i IN-4, IN-5, IN-7, d IN-8 with 4.08, 4.12, 4.17, d4.34%testigccurcyt190,250,130,d40euro with22,1,3,d1dely.itermsofsptillysisthe top best five results show percetge testig MAE 3.98, 4.08, 4.12, 4.17, d 4.34% of positios 13(7), 21(2), 22(5), 31(3), d 32(6) cosiderig the euros 90, 190, 250, 130, d 40, respectively. This result shows tht positio does ot give cler future of sptil oriettio of rditio performce. O the other hd temporl lysis shows cler ptter betwee iput d output with respect to differet yers of dt. I the tble, top best results re from 2012, 2012, 2012, 2012, d 2012, respectively. If compred with the worse resultswhichretestigmaeerrors98.39,99.64,100.87, , d of yers 2003, 2003, 2004, 2003, d 2003, cosiderigeuros40,160,40,10,d160,respectively, it clerly shows tht best result performed by ANN model depeds o closer yer of dt similrly like dely cse Model Behvior with Multiple Trsfer Fuctios (Activtio Fuctios). I this study, differet ctivtio fuctios re used s show i Figure 9 tht deped o differet umber of itertios for comprig their performces o rditio dt. For every stdrd ctivtio fuctio, we used the umber of euros i the hidde lyer s metioed i euro sectio for differet iputs show i Figure 8. After presetig i Tble 3 differet performce prmeters,

10 10 Jourl of Solr Eergy Number of iputs Tble 3: Rditio forecstig; sptil compriso of ANN models with time dely, euros d trsfer fuctio. %performce (MAE) %triig %forecstig Triig Vlidtio Testig MSE RMSE MSE RMSE Yer Dely Neuros Trsfer fuctio Colum Row Rkig stlis compet hrdlims hrdlim pureli compet pureli stli posli logsig pureli tsig posli hrdlims posli compet stlis stlis pureli pureli stli posli logsig tsig stli hrdlims tsig stli pureli pureli posli stlis compet compet hrdlims tsig hrdlims tsig compet logsig posli hrdlim hrdlim tsig hrdlims compet stli

11 Jourl of Solr Eergy 11 Number of iputs Tble 3: Cotiued. %performce (MAE) %triig %forecstig Yer Dely Neuros Trsfer Triig Vlidtio Testig MSE RMSE MSE RMSE fuctio Colum Row Rkig compet tsig pureli compet compet pureli stlis logsig logsig tsig tsig posli tsig stli posli logsig hrdlim logsig stlis tsig logsig hrdlim posli compet iterprettios of their results will follow here. We used the differet umber of delys i the first sectio d differet euros i the secod sectio d to compre differet iputs with differeces ctivtio fuctios re described here. Accordig to the first grph from Figure 10 geerted by the ll iputs re compre with the differet ctivtio fuctios. However, ech iput performs sigifictly differet behvior with its oe kid of fuctio. Stlis ctivtio fuctio results perform the most successful oe, for ll the test prmeters. However, the ccurcy of the iputs ws totlly differet i triig d testig cses; higher ccurte result (lowest MAE vlue) shows the iverse effect durig testig cse. This my be the cse of over- or uderfittig of model with dt. The rest of the fuctios used i this study were ot successful d ccurte eough i group. Tble 3 shows logsig fuctio with IN-69 reportig the best ctivtio fuctio for triig. However, testig results show tht the ccurcy of stlis ctivtio fuctio with IN-11 ws much better. This situtio explis tht totl me bsolute error (MAE) ccordig to itertios cot determie the etwork ccurcy.hece,weobtitherelccurcybsedotestig results. Tble 3 shows the error for ech ctivtio fuctio for differet umbers of iputs, which vry from 1 to 72 fter removig the lst two yers (2003 d 2004) of dt due to GHI (W/m 2 ) Solr rditio plot: forecsted versus ctul Time (hours) differet iputs Figure 10: Rditio forecstig; sptil compriso of ANN models durig triig with time dely, euros, d trsfer fuctio. its poor performce. I terms of sptil lysis top best five percetge MAE testig results show 3.04, 3.08, 3.13, 3.25, d 3.29% of positios 12(4), 31(3), 23(8), 33(9), d 23(8) with trsfer fuctios stlis, compet, hrdlims, hrdlim,

12 12 Jourl of Solr Eergy d pureli, respectively. However, compre with the worse results, which re MAE test error 6.68, 6.69, 6.76, 6.9, d 7.31% of positio 32(5), 31(7), 11(1), 22(5), d 33(9) with trsfer fuctio tsig, logsig, hrdlim, posli, d compet, respectively. This result shows o specil ptter relted to sptil positio; eve the the trget positio is 22(5) ot directly relted to the sme positio 22(5) of iput i terms of performce; therefore it is importt to cosider the eighbor positio i the modelig of ANN. As it is ot direct reltio betwee iput d output irrespective of positio, the modelig eeds expert supervisio. O the other hd temporl lysis shows cler ptter betwee iput d output with respect to differet yers of dt. I the tble, top best results re from 2011, 2012, 2012, 2012, d 2011, respectively. If compred with worse five results which re 2005,2005,2005,2005,d2005,respectively,itclerlyshows tht the best result performed by ANN model depeds o much closer yer of dt similrly like dely d euro cse. Besides the best performce the worst cses result shows tht old yers of dt d lower euro umber do ot ply importtroleibetterperformce. 5. Coclusio This documet icludes the modelig chrcteristic of rtificil eurl etworks bsed o sptil feture. The estimted model ws iitited d tested o solr rditio dt. The results re evluted with differet sttisticl error. This documet certifies the bility of ANN to ccurtely reproduce hour s globl rditio forecst. The estimtio ccurcy of the hourly solr rditio c be chieved by usig covetiol meteorologicl dt of te yers. This model hs bee used extesively for the specific pplictio; due to the dymic ture, modelig eeds professiol dvice. Thissectiogivesmorevluetotheprmeterthtshows the progress of the ANN rchitecture, the dely, euros, d the correspodig trsfer fuctio of sptil positio (Figure 5). The results show high degree of flexibility i the choice of differet iputs d coected prmeters for comprtive ccurcy. Coflict of Iterests The uthors declre tht there is o coflict of iterests regrdig the publictio of this pper. Refereces [1] R. Mrquez d C. F. M. Coimbr, Itr-hour DNI forecstig bsed o cloud trckig imge lysis, Solr Eergy, vol.91, pp , [2] M. J. Ahmd d G. N. Tiwri, Solr rditio models- review, Itertiol Jourl of Eergy Reserch,vol.35,o.4, pp ,2011. [3] V. Bdescu, Correltios to estimte mothly me dily solr globl irrditio: pplictio to Romi, Eergy,vol.24,o. 10, pp , [4] K. Bkirci, Models of solr rditio with hours of bright sushie: review, Reewble d Sustible Eergy Reviews, vol. 13, o. 9, pp , [5] T. Khtib, A. Mohmed, K. Sopi, d M. 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